The VXO

[Bton-FM]

For the price of an obscure crystal you could get yourself a whole tube of TSA5511s! It's a no-brainer, to me at least.

[Zozo]

For the price of an obscure crystal you could get yourself a whole tube of TSA5511s! It's a no-brainer, to me at least.

Yes it's defiantly been interesting and it would appear the VXO is no-ones "Ockham's razor" on here. I think the areas that have been highlighted the most would be frequency agility, then followed by modulation for FM mode, and then finally ordering crystals.

I'm not in favor of a VXO for FM mode these days, but it did have it's place for a while in some applications before synthesizer IC's came about.

EDIT

Maybe it's time for another topic, A transmitter with things that glow perhaps

[Bton-FM]

For the price of an obscure crystal you could get yourself a whole tube of TSA5511s! It's a no-brainer, to me at least.

Yes it's defiantly been interesting and it would appear the VXO is no-ones "Ockham's razor" on here. I think the areas that have been highlighted the most would be frequency agility, then followed by modulation for FM mode, and then finally ordering crystals.

I'm not in favor of a VXO for FM mode these days, but it did have it's place for a while in some applications before synthesizer IC's came about.

Before there was CMOS logic chips available, that must have been the only way to do it without drifting all over the shop! That's proper old school though. The first PLLs using CMOS chips must have been in the 70s?

That said, the synthesizer IC's most builders are using are from the 80s - 90s! The SAA1057 is circa 1983! We are still using very old tech.

[Zozo]

For the price of an obscure crystal you could get yourself a whole tube of TSA5511s! It's a no-brainer, to me at least.

Yes it's defiantly been interesting and it would appear the VXO is no-ones "Ockham's razor" on here. I think the areas that have been highlighted the most would be frequency agility, then followed by modulation for FM mode, and then finally ordering crystals.

I'm not in favor of a VXO for FM mode these days, but it did have it's place for a while in some applications before synthesizer IC's came about.

Before there was CMOS logic chips available, that must have been the only way to do it without drifting all over the shop! That's proper old school though. The first PLLs using CMOS chips must have been in the 70s?

That said, the synthesizer IC's most builders are using are from the 80s - 90s! The SAA1057 is circa 1983! We are still using very old tech.

I guess your right about the CMOS logic that was possibly used in Broadcast Spec Equipment, However I've not had much exposure to that kind of tech from that era to be completely honest. The transmitting equipment from the 1970's that I've either seen, used or owned was all crystal controlled oscillators for either AM/ FM or SSB modes, and not used in broadcasting.

Yes the SAA1057, you know I think I have just one of those left in a bin on the racking. I doubt I'll ever use it. Maybe sling it up on eBay.

EDIT

Any idea how many posts I need to make to get rid of that "Who u callin ne guy bruv" thing.

[jvok]

I've read before about the old Marconi rigs the BBC used when they first started on FM in the 50s. They used a really low frequency crystal (like single digit MHz) and then loads of cascaded doublers/triplers. Gets you wide/linear modulation out of a crystal. Sounds like a pita to align but I guess it was the only way before PLLs. And if you're the BBC it's not like you'd ever have to retune it.

On the topic of what makes a PLL stable: at low frequency (within the loop bandwidth) it's purely controlled by the reference crystal. Once you get past the loop bandwidth then the VCO is on its own and the PLL doesn't make any difference.

So the crystal controls your long term drift (seconds, minutes, hours...) as you'd expect. Then the VCO controls the short term phase noise, which you'd just hear as hiss on the receiver. That's why low phase noise VCO designs matter - it won't affect stability but it does make your signal cleaner.

Of course that's all only true as long as the PLL stays locked. If your VCO is drifting that badly then obviously a PLL won't save you. But as long as it stays in the locking range then long term VCO drift (heating effects etc.) doesn't make any difference.

[jvok]

Quick reply about that LPF: it's not really an LPF at all. That parallel tuned circuit might stop the 2nd harmonic but at higher frequencies it's impedance drops again so it just looks like a short circuit. If you think about it the coil is a high impedance but the cap is a short. You put a short in parallel with a high impedance and you get... a short. So your high harmonics (3rd, 4th etc) just go straight through.

The caps to ground will give you a bit of an LPF effect but not much. So I'd say it's more HTF than LPF.

[Albert H]

Another point which makes me favor VXO's is there's less components needed, No synthesizer IC or supporting controller. You can even put the code writing to one side. One of the only down sides to the VXO was ordering Crystals, But now you can order one very cheaply and they get cheaper as you increase you order quantity.(Established pirates long term investment maybe).

There are three gigantic problems with a VXO - the need for a specific crystal (quite expensive and "unusual" frequencies engender unwanted interest), and very poor modulation linearity. With a VXO you're "pulling" the crystal off its natural frequency. The deviation from the centre frequency is simply NOT linear. Back when a Big Broadcasting Concern used VXO VHF rigs, they had to use "pre-distortion" networks to correct for the non-linearity, which didn't work particularly well. The third problem (related to the second) is that the available deviation is limited.

VXOs are fine for voice communication, but no good for high quality FM audio, in my experience.

The deviation problems are why PLLs are generally used for VHF high quality audio transmitters. You can use cheap, standard crystals - I pay around £0.26 for the ones I use - and the ICs and diodes for a basic PLL cost around £1.40 if you shop carefully. If you make the PLL in SMD, it can be tiny, and if you put the crystal in an oven, you can get spectacular stability. If you're being really flash, you can receive 198kHz off-air, square it up and divide it to somewhere useful as the reference for your PLL. The frequency of your rig is then controlled by the frequency source at the National Physical Laboratory!

The inspectors from OFCOM complained that a MW RSL I did was a few Hz off-channel - I demonstrated that the problem lay with their inaccurate frequency counter, not my NPL-referred rig!

Actual rig stability is often a function of the PLL loop filter. Roger Howe had a problem with one of "his" designs that "hunted" around the required frequency. The problem was actually down to Roger incorrectly noting down the time constant components in my PLL circuit at the time......

Way back, there were three or four predominant exciter designs:

The first came from the 1970s, which was a doubler-doubler circuit, and had a bizarre "totem pole" coil assembly in the middle of a large square PCB. It used five RF transistors, one more (low noise one) for the modulator, and a couple for zener-based regulators. It was a free-running VFO design, and was capable of good stability if carefully constructed. It was used as the exciter for a lot of Radio Invicta QQV06-40 rigs. The circuit originated from "The Tadworth Mouth".

The second one was another doubler-doubler (later simplified to a tripler variant) that was used for the free-running RFL rigs, and widely coped. The circuit originated from "Michael Martin", and used a Colpitts oscillator with silvered-mica capacitors for stability. The doubler stages used under-biased BSX20 transistors with high-Q collector circuits. The doublers had to be screened from each other to prevent spurs. The original circuit used oscillator, buffer, doubler, doubler, filter / amplifier, amplifier to about 100mW, all using BSX20s (6 stages), followed by a 2N4427 for about a Watt. If built carefully, with the right components, it was reasonably clean and quite stable.

BDHK simplified the Michael Martin circuit so that the oscillator ran at one third of the output frequency, then used an un-tuned buffer stage, a tripler, and an amplifier to 100mW - reducing the circuit to 4 BSX20s and a 2N4427.

The third circuit came from the Netherlands, and used a push-pull oscillator (similar to the NRG oscillator, but with only two coils), and a push-pull doubler / amplifier stage to about ½W. My version of it then had a pair of SD1127s (or MRF237s) for about 7 Watts. That was enough to drive a 2N6084 to over 40 Watts if critically tuned! This was the first circuit that I added a (TTL) PLL to for best stability.

The last circuit was a kludge based on a circuit from Radcom magazine, and used two transistors in a sort of "cascode" affair, a doubler with two tuned circuits, an amplifier stage with a toroidal collector transformer to drive the base of a 2N4427. This formed the basis of the BW exciters. It wasn't a bad circuit, but the BW insistence on using a single varicap, and tuning the whole band led to insanely high mod sensitivity, and a propensity for nasty hum, due to RF feedback. Roger did a lot of work on getting the output clean, and for that reason abandoned the doubler, moving to an at-frequency oscillator. This worsened the RF feedback problem, of course. The BW boxes used every trick in the book to try to minimise the RF feedback problems - often rather unsuccessfully.

There were a couple of other designs - the "Thameside" exciter used an RF op-amp as its buffer stage, and I did one that used the SL560 RF amplifier chip as oscillator and buffer in one, and a second SL560 as doubler and filter amp. My 560 exciter also used the SP8629 as the prescaler from Band II (or Band III for link gear), and then used cheap CMOS for the rest of the PLL at low frequencies (around 1 MHz). My exciter looked a bit weird, with three 8-pin DIL ICs and a 4427 at the end. The CMOS PLL bits were beneath the board, with the chip pins bent outwards to make them into "big surface-mounted" parts. I bought a huge boxful of SL560s from a component broker for about £0.02 each and had to use them up!

Variants of the RFL and BW circuits are being made to this day. The latest iteration of the RFL circuit that I've seen uses a half-frequency oscillator and an un-tuned diode doubler (so that it's broadband) into a two-stage amplifier for around a Watt.

The final exciter design I've seen recently uses a 2N4427 as an at-frequency "power oscillator", driving a RD15 FET for over 12 Watts in just two stages!

[Zozo]

VXOs are fine for voice communication, but no good for high quality FM audio, in my experience.

I agree fully with that statement as its perfectly expectable for say the 2m band with the limitations in deviation. But wide enough for an intelligible transmission.

Variants of the RFL and BW circuits are being made to this day. The latest iteration of the RFL circuit that I've seen uses a half-frequency oscillator and an un-tuned diode doubler (so that it's broadband) into a two-stage amplifier for around a Watt.

I be interested to see an example schematic of the diode doubler. I may try to do something like this for a Homebrew 2m TX.

I do like the VXO for use with AM designs, especially in valve gear. I'd think that modern solid state synthesizer inside valve designs looks really out of place, But that's probably just me being a purest for vintage designs. However I must concede to using a solid state rectifier in valve gear, but that's probably as far as I will go.

I was never really a fan of the Hybrid stuff, which was all solid state apart from the final PA. But AM sound horrible on those in my opinion. In fact even some of the modern expensive "Rice Boxes" today sound terrible on AM. But SSB and FM is fine. It's like AM was an after thought.

[jvok]

I be interested to see an example schematic of the diode doubler. I may try to do something like this for a Homebrew 2m TX.

Here's where I first saw it - not FM but same shit and same frequencies. http://ludens.cl/Electron/converter/converter.html

Doubler is L1/D1/D2. It's basically your classic two diode rectifier circuit but fed with RF instead of 50Hz. Needs fast diodes obviously - he used expensive HP2800s but cheap RF schottkys like 1N5711 works fine. Worst part about it imo is that you need the transformer to get the two phase input, but if you wind your own well magnet wire and cores are cheap. I used T50-6 powdered iron because I had it in stock but at a guess type 67 ferrite would work too.

That schematic also has another transformer at the output but you don't need it. Loading it with a resistor to ground then coupling the output with a cap works just fine.

IMO it works better than using a transistor as a doubler. Less 1/2f feedthrough. But be warned for best results you need a good clean sine wave. Anything asymetrical in your input will cause 1/2f.

I'm working on a rig based on this at the moment. If I get it to work then I'll post the circuit.

[Zozo]

I be interested to see an example schematic of the diode doubler. I may try to do something like this for a Homebrew 2m TX.

Here's where I first saw it - not FM but same shit and same frequencies. http://ludens.cl/Electron/converter/converter.html

Doubler is L1/D1/D2. It's basically your classic two diode rectifier circuit but fed with RF instead of 50Hz. Needs fast diodes obviously - he used expensive HP2800s but cheap RF schottkys like 1N5711 works fine. Worst part about it imo is that you need the transformer to get the two phase input, but if you wind your own well magnet wire and cores are cheap. I used T50-6 powdered iron because I had it in stock but at a guess type 67 ferrite would work too.

That schematic also has another transformer at the output but you don't need it. Loading it with a resistor to ground then coupling the output with a cap works just fine.

IMO it works better than using a transistor as a doubler. Less 1/2f feedthrough. But be warned for best results you need a good clean sine wave. Anything asymetrical in your input will cause 1/2f.

I'm working on a rig based on this at the moment. If I get it to work then I'll post the circuit.

Thanks Jvok for the link, I've only had a quick glance and noticed what looks like a Diode Ring. I'll take another look in the morning as my brain is in natural at this time of night "H.I."

Look forward to seeing you circuit design when its ready. 73's

[sinus trouble]

Any idea how many posts I need to make to get rid of that "Who u callin ne guy bruv" thing.

You could just spam the Forum?...... But you will most likely get Banned!

[teckniqs]

Just another 14 more posts and this ne guy will become big in da game.. trust

[Zozo]

Just another 14 more posts and this ne guy will become big in da game.. trust

Ok thanks techniqs

[Bton-FM]

Any idea how many posts I need to make to get rid of that "Who u callin ne guy bruv" thing.

You could just spam the Forum?...... But you will most likely get Banned!

On the plus side it would mean I could deploy the smiley for once!

[sinus trouble]

Just another 14 more posts and this ne guy will become big in da game.. trust

Big Man Tings Ya Get Me!

[sinus trouble]

Any idea how many posts I need to make to get rid of that "Who u callin ne guy bruv" thing.

You could just spam the Forum?...... But you will most likely get Banned!

On the plus side it would mean I could deploy the smiley for once!

[Krakatoa]

I like vintage RF circuits, but to not take advantage of today's technology is like living in the past.
To make a very simple vxo and frequency multiplier all in one chip that you can even use as a narrowband FM spy microphone, there is the ICS501, capable of generating up to 160MHz from a standard quartz, using the available multiplying factors it has in its internal PLL.
Given that the output is digital (square wave) is rich in harmonics and you can choose to filter the fundamental or continue multiplying with external circuits up to the UHF range.
Here is an example in a russian blog:
https://vrtp.ru/index.php?showtopic=27258&st=30

[Zozo]

I like vintage RF circuits, but to not take advantage of today's technology is like living in the past.
To make a very simple vxo and frequency multiplier all in one chip that you can even use as a narrowband FM spy microphone, there is the ICS501, capable of generating up to 160MHz from a standard quartz, using the available multiplying factors it has in its internal PLL.
Given that the output is digital (square wave) is rich in harmonics and you can choose to filter the fundamental or continue multiplying with external circuits up to the UHF range.
Here is an example in a russian blog:
https://vrtp.ru/index.php?showtopic=27258&st=30

Thanks krakatoa for the link which I found very interesting. I like the simplicity of the design. Also I think we share something in common as you say you like vintage RF circuits too. I'm defiantly living in the past being a Medium Wave pirate fanatic, in fact any band where AM Mode is permitted. I also prefer valve designs, solid state is great and it's not like I avoid it. But it's just not the same with things that glow.

There's a lot of topics on that other forum which I found interesting also. Might be worth registering on there too.

So far this topic has primarily been focused on FM Mode, But there's just one more area left to cover regarding the VXO.

AM 48Mtr Band VXO Driver.

SW48M1PEP.png

There's so many Xtals available for that band, New and Old it's just hard not to build one of these.