Saturday, 29 April 2017

UHF Travels

Many amateur experimenters stay in the familiar comfort zone of HF. A small number venture into VHF, and even fewer devote time designing, building and operating for the UHF & microwave spectrums. 

Outside of amateur radio, broadband UHF radio operation dominates the airwaves:  data or voice communications on smart phones hum along nonstop on WAN, LAN, cellular, PCS, PSTN and satellite networks.  Then too, we’ve got the ISM bands that backbone industry and commerce @ UHF and higher spectrums for multiple groups, including radio astronomers. 

Also digital television broadcasting standards may involve the UHF spectrum, and so, a pattern emerges: UHF circuitry sending data that's controlled by CPUs.

This UHF proliferation benefits us experimenters by providing an excellent bevy of fast parts such as 25+ GHz transistors, cheaper, more abundant transmission line + connector choices, numerous scientific articles, faster + cheaper test equipment --- and loads of inspiration. To boot, many transmitting circuits are low voltage and/or low power – aka “QRP”. This is the new QRP !  Fresh, relevant, science-driven -- and supporting the marriage of code plus RF design in the new age via the latest tools, techniques and hardware.

The physics and challenges of RF design haven't changed however. For example, the Smith Chart still works perfectly, and measurement techniques remain the same. However, some differences become quickly obvious as you plow in: 1 stark example == resonator Q. Vitamin Q gets hard to come by as we move up in frequency. Further, the ease + slack and forgiveness of HF quietly disappears and this proves both vexing and challenging to us builders.

Casual construction techniques work poorly at UHF. Consider a Class A common emitter BJT amplifier: A poorly RF grounded emitter lead may add parasitic inductance that results in series feedback which degenerates the signal and reduces amplifier gain.

Further, I’ve learned that a high bandwidth transistor amp designed and/or built poorly make frustrating oscillators, or, just offers low gain due to mismatch or instability. Unsurprisingly RF filters with poor RF grounding @ UHF poorly attenuate UHF.  I'll show this graphically in the next section.

UHF design looks scary to some: you have to master the Smith Chart, obtain some fast transistors, and learn about microstrip techniques and so on.

Then comes the task of making your test bench.  Yikes!

High bandwidth gear may equal high cost depending on what you build or purchase. For sure, you’ll need some form of a 50 Ω detector. My main UHF detector = a 3 GHz spectrum analyzer with a tracking generator. This allows me to measure with or without sweeping 1 or 2 port circuits. You also get a frequency counter in the RG + SA package -- so it seems like a good choice; at least for now.

I also designed and built a wide band return-loss bridge and plan to improve this design over time. I would love to own a network analyzer, but will hold off until I find an affordable unit with a bandwidth >= 10 GHz.  10 GHz?  Yes, I hope to work 10 GHz EME using JTF4 mode with Doppler correction on homebrew gear 1 day.

My eventual goal @ UHF -- to design and build a UHF radio astronomy system looms in the background, however, first, I must learn to design and match amplifiers, tackle low phase noise, temperature-stable oscillators and learn how to make filters that work well into microwave.

I’ll blog some of my experiments starting today.

[1] Ugly Construction

RF signals follow the path of least impedance. This means plying double-sided copper clad board with the lower half serving as the microstrip ground plane. Currently, I use glass-fiber epoxy laminate FR-4, 1 ounce copper boards with 1.37 mil trace thickness and a substrate height of 1.6mm (1/16 inch).

Above — I got a deal on some MG-Chemicals FR-4 and will use it for all of my 2017 experiments.  FR-4 is common, easily available and cost effective.
Permittivity, or the dielectric constant Er =  ranges between batches and manufacturers of FR-4. I've read Er values from 3.9 to 4.8 and tend to use the standard 4.7. This means a 50 Ω stripline gets cut to a 2.9mm width.

Although, substrate thickness varies and insertion loss increase with frequency, I think we can manage OK with FR-4 copper clad laminate board at UHF.  PTFE-based boards pose an option if we require critical impedances , λ fractions, or lower insertion loss as you move up in frequency.

Above — 4 square feet of FR-4 laid out on the lawn. These boards will hopefully bear some good experiments ahead.

 RF Grounding 

Above — A portion of an older, beat-up 12 inch by 12 inch 40 gauge copper sheet I use to connect the copper surfaces at all board edges. I just cut it with scissors. You can also smooth out the pieces before, or after cutting them.

I get this sheet copper from Monte Allums in the USA. Click for his link. I've also built tops for shielded RF boxes and resonator dividers with this stuff. Oh, and I shielded an electric guitar control cavity for someone using this Cu as well.

Above — An example edge dressing. We start with a small 2-sided piece of FR-4 and a cutting of Montee Allums 40 gauge copper sheet.

Above — I' ve folded the copper sheet over 1 edge. Looking a little uneven, it's clearly not my best effort, but on the other hand -- it will work fine. I scraped the copper sheet with the edge of the scissors so it lies flush against the FR-4 with no air gaps. Time to solder.

Above — The copper sheet gets wrapped around soldered on both sides. Then repeat for the other 3 FR-4 sides. When completed, you'll have a board ready for carving out strip lines or applying classic ugly construction when using B+ posts, connectors and capacitors as stand-offs.

A 2-sided FR-4 board with copper strips joining all edges will provide stalwart RF grounding from DC to VHF. For UHF we need to drill via holes and connect the 2 surfaces with copper wires strategically located wherever UHF ground is needed.

I'll demonstrate why with an actual experiment based on a 7- element low-pass filter that I designed to use the lumped-element inductors and capacitors I had on hand.

The inductors = Coilcraft 1008CS series wire wound on a ceramic chips. These are in size 0805. Click for a datasheet. I've since stocked many values of these amazing parts in both size 0805 and 0603.

Above — Simulation of my filter in GPLA, a program that ships with EMRFD.

I then built a filter, but did not solder the copper edges together with Cu sheet metal and just use 1 via wire for the RF ground on each of the 4 caps.

Above — A sweep out to 3 GHz shows sub-optimal RF grounding leads to poor attenuation above ~1.5 GHz. This = a terrible filter.

Above — A sweep made after adding the copper edges and 1 more via wire to each of the 4 capacitors. While better than previous, attenuation poops out at 2.475 GHz.

Above — Final version swept after adding 2 more via wires to each of the 4 capacitors. Clearly soldering 4 copper via wires per ground node boosted the filter's attenuation at 3 GHz.

Above — I purchased a popular 400-470 MHz low-pass filter sold on eBay that sells for ~ $10.00. At 2.81 GHz, the filter's stop band just falls apart. Further, the 3 dB cut-off of my particular filter = 580 MHz; much higher than advertised.

Above — The low-pass filter swept above.

Above — A MCL PLP-550 low-pass filter. This older filter suffered bent pins and I failed to get a short path for the 6 ground pins to the ground plane on the lower half of the board. 1 mm of wire length potentially could = 1 nH.

Above — Poor RF ground at UHF trashed the upper stop band of my poorly mounted MCL PLP-550 low-pass filter.

Above  — I attempted to reduce the ground path lengths and also placed 2 via wires by each SMA connector. This boosted the UHF stop band somewhat. I'll take an entirely different approach when I redo this filter's breadboard 1 day.

I've learned the bitter truth about RF ground -- and now RF ground lies foremost in my mind when I breadboard a circuit.

Multiple, short, low inductance via wires provide a low-impedance connection
to the ground plane below and in addition to improving RF bypass may reduce noise + interference.

[2] SAW based Oscillators

SAW resonators offer an easy way to generate a single frequency with a decent loaded Q to reduce oscillator single sideband phase noise when compared to LC, or varactor-tuned circuits @ UHF.

It's also possible to tune them as a VCSO ( voltage controlled SAW oscillator). They see use in many commercial items such as car door openers, chemical sensors and even wearable medical devices. You'll often view them in datasheets covering the range of 200 - 1200 MHz, although other frequencies are also available. The unloaded Q of a typical SAW device lies between 6 - 12K.  That's much better than that of a VCO with the inductor cut out in the copper board (Q = 50 - 100) and tuned with varactor(s).

Of course, the variable SAW resonator lacks the wide tuning bandwidth needed for many projects -- UHF work proves all about compromise.

So far, I've built 7 or 8 SAW oscillators and many were crappy. Some of the problems included oscillation at an overtone, low output, parasitic oscillation, and not running at the correct SAW frequency.

I'll show an oscillator I built to use for OIP3 measurements of BJT amplifiers using my lab-grade HP signal source as the other tone for these 2 tone measures.

Above  — My first attempt at a feedback loop oscillator. Normally, we build negative resistance type oscillators.

Above  — Here's my bread board layout diagram for the first and second stages. I carved these signal paths into the topmost board with a motorized cutting tool.

Above  —The output of the feedback loop oscillator. If you take the signal from the collector, the AC voltage may run as high as 1.5 Vpp, but it's rich with harmonic energy. I sampled some form the transistor base via a 0.5 pF capacitor. All caps < 100n were C0G temperature coefficient. Transistors such as the 2SC3583 offer tremendous gain at HF, so HF and even AF bypass is needed to keep circuitry stable.

Above  — Output of the second transistor stage. I'll comment about the collector feedback bias in a later blog post.  I attempted to match the output impedance to the 50 ohm input impedance of the MMIC, I did OK, but failed to account for stray reactances and also I didn't have caps less than 0.5 pF at the time. The 33 Ω resistor in series with the 22n lower its Q to stabilize the amplifier.

Above  — Output of the final stage, a MMIC. I lost significant signal amplitude because my poor filter layout. My carved microstrip paths were too long and so, acted like inductors that detuned the filter and created some reflections. Of course, you never know this until after you've carved your breadboard. Still, this circuit works OK for amplifier, or mixer 2-tone measures and I learned a lot by designing and making it.

This was by far the most temperature stable SAW oscillator I've built.

Above  — An experimental negative resistance oscillator that allowed the selection of 1 of 3 SAW resonators. It worked to a point. The problems encountered are shown in the inlay.

Above  — An experimental Butler SAW oscillator. When you remove the SAW resonator, the circuit still oscillated and needs to be within 5% of the SAW value or the frequency stability suffers. Further, the oscillator worked best with a tuned collector. I need to work on this some more. It doesn't take much to push this circuit into an overtone oscillator; intentional or not.
I'm always trying to boost the QL since SSB phase noise lies inversely proportional to the square of the loaded Q.

Above  — The output signal with a tuned collector. Oscillators combine art, science and to some extent, a little luck.

[3] RF Amplifier

I'll show 1 experimental amp, since I'm still learning through experiments and this blog post is running long in the tooth.

Above  — A 1 GHz amp matched by the the brute-force method: The input and output are 50 Ω microstrip line slightly greater than λ /2 long ( accounting for the FR-4 velocity factor ). To tune the input and output, you slide a size 0805 capacitor along that's attached to a thick toothpick, find the sweet spot and then tune a variable cap right at the sweet spot. The variable cap then gets removed, measured, and a nearest standard value cap goes in place.

I chose a 2SC3583 BJT with a VCE of 8 volts and 5 mA collector current. Using datasheet S parameters I calculated the maximal available gain at 15.98 dB -- and also the maximum stable gain at 13.4 dB. In the breadboard, I got 11.2 dB gain, which seemed reasonable for my first, crude 1 GHz RF amp.


Above  — The 1 GHz breadboard. It's too long for practical purposes, but not for learning.


Above  — My single best UHF reference book. Out of print.


Saturday, 21 January 2017

1968 Princeton Reverb Repairs

1968 Princeton Reverb Repairs — A reprised post from my old QRP HomeBuilder website

This somewhat rare 1968 silver face Princeton Reverb had aluminum trim surrounding the grill cloth. We verified its age by the output transformer code, the trim and by serial number. This particular amp ran a GZ34 rectifier in the rectifier tube socket.

Boy, it sounded terrible!  According to the owner, it sat in a closet for 15 years; but prior to that, some teenagers had borrowed it to "rock it hard", but brought it back and stated that it did not sound very good. After replacing 4 tubes, the power supply capacitors, the bias capacitor, a B+ chain resistor, and the speaker --- this amp sounded pleasant once again.

During physical examination and testing, I found the following problems:

  1. A 4 amp fuse was in the fuse socket
  2. Blown speaker
  3. There were 2 different brands of 6L6 tubes
  4. Loud hum, noise and microphonics in the AF chain
  5. The rectifier tube was audibly clinking and stinking
  6. Low AF output power
  7. The first 1K resistor on the B+ appeared to be temperature distorted
  8. I heard an approximately ~1 KHz intermittent oscillation

Above — Post CBS Princeton Reverb. The Princeton Reverb was essentially a poor man's Deluxe Reverb with a 10 inch speaker. Rated at  around 12 watts, it ran a much smaller output transformer and as a result, offered much less head room than the Deluxe Reverb. The tolex on this old amp looked in incredible shape. All it needed was a good cleaning.

Above — The look on our jazz cat's face says it all. This amp sounded terrible. He
demanded that I make repairs as soon as possible and return it to the owner.

Above — The chassis removed from the wood. The PR chassis proved very compact, lightweight and easy to work on.

Above — A view inside the chassis. This circuit and its schematic looked very much like the blackface pre-CBS version. A few newer, non-Fender parts gave evidence of previous repair(s). This can get scary. Fixing some else's bad repairs may open a Pandora's box. For this particular amp, the previous repairs seemed OK.

Above — The 4 main electrolytic B+ supply capacitors are housed inside 1 can
which is referred to  as a multiple section capacitor. This is the old multiple section capacitor. Note the heat disfigured 1K ohm resistor to the right of the capacitor. I removed it and measured 1K6 ohms.

Above — Removal of an old multi-section capacitor is never fun. I used an 80 watt soldering iron, solder wick and a flat screw driver to unlatch the anchoring tabs from the main chassis. After capacitor removal, the remaining solder was removed and I cleaned and buffed the local chassis area.

Above — The new multiple section capacitor. It's diameter measured 1 - 3/8 inches. This means obtaining a special part as modern cans are greater in diameter and won't fit in a vintage amp. I got this cap from Antique Electronic Supply. The number = C-EC20X4-475 and it's a 20/20/20/20 uF @ 475 VDC. Apparently they're manufactured on original Mallory equipment.

Above — The new multi-section capacitor is soldered in. I replaced the disfigured 1K resistor with a NOS version. It ran cool to touch during testing. For safety purpose, I palpated it immediately after the power was switched off and the high voltage was bled to ground. Nowadays, I measure temperature with an infrared thermometer.

Above — Replacing the bias capacitor removed the intermittent ~1 KHz oscillation and decreased hum. I only had an axial type on hand and it worked fine.

Above — The old and new 6V6 finals are shown. A matched set of Electro-Harmonix 6V6s were installed. I loved the sound of these Russian-built EH 6V6 offerings. I also put in a new, low-noise, Sovtek 12AX7a in the number 1 preamp tube socket. Further, a new Sovtek rectifier tube replaced the clanking old rectifier tube. The other tubes seemed OK.

Above — The old and new 10 inch speakers. The speaker choice was made to suit the owner. He wanted maximum head room and plays only clean guitar. I chose a Fender 099-4810-004. This speaker is actually made by Eminence, a company I like and whose products I have used on other projects. The new speaker sounds great and most importantly, pleases the owner. The old speaker cone and surround were cracked and separated. Further, the cone felt stiff + brittle --- and a new speaker proved the best way to go. Fender didn't put a great speaker in the PR to begin with.

Above —  A rear view of the final tested chassis back in the wood and connected to the new speaker.

Above —  The jazz cat now mesmerized by the sweet tone of the restored Princeton Reverb. The parts total ran about $200 and much of it arose from shipping/handling/duty and tax costs to Canada. The owner only wanted to spend a maximum of $200, so it felt good to come in on budget for a change.

Above — My bench back in 2008. I built and repaired many tube amps here.


Tube amplifiers operate at high DC voltages. Repairing, modifying or building tube amps can be dangerous, or in some cases, fatal.

Thursday, 5 January 2017

Polytone Amplifier Page

This is a reprise of the Polytone Amplifier Page from my old QRP HomeBuilder web site (2008).

Above — The Mega-Brute has an eight inch speaker and is über-portable. This amp tucks into almost any corner. On the rare occasion when I play for a function, I may use a 12 inch extension speaker, depending on the situation.
I've owned 4 different Polytone amplifiers and like their tone and compactness. I must disclaim this blog post by stating that what I have written is just 1 opinion. Discussing guitar amplifiers is akin to stepping into a quagmire. Guitarists often become quite emotional and speak passionately, or possibly even overly-critical when discussing gear. When you consider that most players don't even perform in front of an audience, or record for mass distribution, the "gear wars" diatribe can get really quite silly. If your tone sounds good to you, then perhaps your amp is suitable (at least for this month).

For jazz guitar, there are countless amplifier choices and the list of of good jazz guitar amplifiers has really grown in the last few years. The trend seems to be towards more hi-fi sound (less distortion-more headroom and more power), smaller/lighter designs at somewhat increased cost.

Consider talking to Michael Biller at Sound Island Music if you wish to talk to someone with considerable knowledge and practical experience regarding modern jazz guitar amplification.  Michael stocks many products -- and his passion about helping you obtain your perfect jazz or double bass guitar tone really shines through when you talk to him.

Polytone Mega-Brute Amplifier

Canadian guitarist, Glenn Murch once told me that Polytone guitar amps basically have one sound and you either like it or not. I agree with him. This sound is not
ultra-high in fidelity ( compared to more contemporary designs ), has a dark voicing, a distinct midrange honk and starts to distort at high volume settings. This is exactly why I like Polytone amps in some situations. To each, their own.

The Mega Brute combo amp has an 8 inch speaker. This is probably not the best amp to use if  you play in a big band, but it works okay with a trio if you are happy with the sound it provides and the drummer uses brushes and/or soft hands with sticks.

I have owned Mini-Brutes with 15 inch and 12 inch speakers as well as the Mega-Brute combo amp and head. The various Brute-series amps are worth a trial if you're in the market for a mid-price jazz guitar amp.

Official Polytone Page  Click

Old Murch Music Polytone Page  Click

Old Murch Music Polytone Schematics Page  Click

Above — Rear view. The tolex work is quite excellent. I love closed back speaker
amps for that bass thump, although, in my opinion, ported designs are preferable. The porting seems to be via the via the low input instrument, pre-amp out and FX loop jacks!

Above — Top view showing the various control pots and switches. The sonic circuit was a great addition to the Brute series. The previous Brute design had an overdrive circuit which failed to merit wide acclaim. I personally do not use the sonic circuit and rely upon the main amp circuit. I like the slight break up of the power amp when driven hard, although it is by no means a Marshall-style crunch tone. Baxandall equalizer plus decent spring reverb.

Above — My Mega-Brute atop a 1 by 12 Marshall cabinet. This proved a pleasant
combination for R & B plus jazz-fusion work.

Above — The other Mega-Brute.  An amp head which became known as the
Mega-Brain. This product was discontinued. The speaker is a Raezer's Edge Stealth 12 speaker cabinet. I sold this head in 2004 and now regret it.

Above —  Inside the Raezer's Edge Stealth 12 speaker cabinet. Underlay foam, carpet and fiberglass insulation absorb standing waves + reflections, lower the box Q and hopefully smooths the bass response. This speaker/box sounds warm with a flat response.

Above — Various views inside the Raezer's Edge Stealth 12 speaker cabinet. Well engineered and built.

Above — I like Eminence speakers; including the Eminence Patriot Swamp Thang mounted in another cabinet for my stereo amp rig.

 Above — My old Mini-Brute with a 15 inch speaker on its side ready to carry. Bass for days!