Friday, 22 August 2014

Ramp Up The Amps

After much deliberation, reading and checking, a couple of things occurred to me. First was that I should try varying the emitter resistor in the Wispy LO, to try and increase the drive to the receive mixer. Germanium diodes typically turn on around 0.3V, so with a drive of barely 240mV pk-pk, I doubt they were on enough. The resistor in circuit is 2k2. So I tried tack soldering another 1k in parallel. This brought the level up to closer to 300mV pk-pk. A bit more playing here and I should be able to obtain the right level of drive. It will probably mean though that the Tx chain will need completely realigning!

The other thing that suddenly popped into my head was why my LM386 audio PA seemed to overload so badly when fed from the Wispy audio chain. I suddenly thought, perhaps I had the gain control pot wired wrong? Indeed I had! So after changing the wiper of the pot to go to the input capacitor and not the input pin of the chip, all should now be well.

My little self contained LM386 audio power amplifier
Oooh! Look, Ive just realised how to add captions to my pictures! Expect more of that!

Emperor Mong Defeated!

Well, It seems my problems with the Wispy's audio stage have been to an unannounced visit to my workshop by the Emperor Mong. That and a crappy earth or earth loop between the test set and the variable PSU!

Using Roger G3XBM's AF strip design as detailed in my last post, and listening with a crystal earpiece, I set the test set to about -75dBm on a 28.1261MHz carrier. At first there was nowt but buzzing! Realising that this was probably an earth issue, I switched the supply to the AF strip and my mock-up for 'TennaLady' to a 12v SLAB. I now had a clean output but no tone! I again had to back the input right, right down, this time to nearly -20dBm!!!

Alas, It was but the Emperor, having one last play with me! I decided to swap the port on the test set, to see if that made a difference, and discovered I was feeding the project with the BNC to Croc clip cable, but the test set was connected to a BNC-BNC lead! I quickly swapped to the correct cable, and nearly had my ear blow out!

Reducing the signal level, after adjusting the frequency to something that gave a nice to listen to tone, I was able to go right down to -115dBm before I had to start on/off comparisons!

By doing on/off (switching on and off the RF output of the test set) I discovered that I could easily discern a -120dBm signal, and could just make it out down to -125dBm.

So, the audio strip works as expected. However, as far as the receiver goes, this is using my SBL-1 DBM based 'TennaLady' mock up, not the Polyakov mixer based Wispy. I will try the Wispy receiver chain later, after trying the AF stage hooked up to an LM386 based audio PA, which hopefully will drive a small loudspeaker.

Wispy AF strip rebuilt

The two transistor (common base - common emitter) audio amplifier strip for the Wispy transceiver beacon has been rebuilt as a self contained little module on a small precut piece of Veroboard. This makes a nice looking little module, especially since I took the time and trouble to add proper PCB pins for the external connections!

Common Base - Common Emitter AF amp

I have to confess however that the layout is not as compact as I had planned, due to an error in designing it. But it fits neatly on the little precut piece of board.

Following on from discussion last night with Roger about how he checked the audio level, I connected a high impedance crystal earpiece to this amp, and then fed it with an AF signal at 800Hz from my Marconi 2955. I could quite clearly hear the test sets 1mV signal, along with a lot of noise and other switching artifacts from the test set! (They're not the cleanest of machines!). Knowing i'm looking for about -120dBm sensitivity on the receiver, which equates to about 0.2uV, I decided to see if I could go lower. So, another previous project was put in line -

Homebrew 50 ohm Attenuator Bank

Now, this is of course designed for 50 ohm use, so some inaccuracy has to be expected, plus I'm relying on my own ears! But I found I could still make out the tone even with a further 28dB of attenuation switched in (possibly more, but thats the most I can be confident at), which is a signal level into the amp of around -95dBm, or ~4uV. I suspect with a cleaner input it could be lower.

Other bits of electronics on the go today included a 10m long control cable for my LDG RT-11 Auto remote ATU, this required soldering a pair of 9-way D-types, which is always a fiddly job. Ive also added a brass rod and plate (made from a 2p piece) to the gate input of the FET electroscope, which has tamed the beast somewhat!

Next task is to add more gain to the AF strip, to allow the direct drive of a small loudspeaker. I already have a small general purpose audio power amp for this, based on an LM386 chip. That should complete the audio stages of the Wispy project, and allow me then to move to the mixer again.

Thursday, 21 August 2014

Wispy receiver - really as bad as I think?

A thought occurred to me a while ago - is my Wispy receiver really as bad as I think, or is there something else ive forgotten to take account of?

Well, using my Marconi 2955, I get no noticeable reception until -50dBm, ish. BUT - Im using the test sets own 'scope and audio system, or my Hitachi 212 oscilloscope, what if the audio level from the receive chain is too low for these? Might it be that the level from the AF stages is not sufficient to drive either of them until this input level?

Somewhere, I read about how Roger determined his sensitivity figures, which were done much like mine by ear, but I think Roger listened directly to the output of the AF strip. Now, did he use headphones, or perhaps a Hi-Z crystal earpiece? Is this my problem - I dont have enough audio output to actually listen to to determine the sensitivity?

Im going to rebuild the audio strip as a self contained module on Veroboard. I can then test it to see how it works, and either feed an earpiece, or use it to drive another amp capable of giving enough level for a small loudspeaker.

Getting Nowhere - Time to Divide and Conquer

No matter what I try recently, Wispy will not work on receive for me. I seem not to have enough LO drive, but the buffer amp i've tried doesn't work! Likewise, I cant seem to get anywhere with the receiver aspect of 'TennaLady'.

It doesnt help that there could be issues with my test equipment again, not easy to prove one way or the other!

Ive also been playing with what should have been a simple 'battery low' indicator using a PNP transistor and zener diode, but even this wouldnt work! I do at least know now why this was the case, and have an appreciation of how such a simple circuit isnt necessarily the best solution.

One thing I have built successfully though, although it doesnt work quite how i'd like for its actual intended purpose, is a FET Electroscope. This amazingly simple device simply detects static fields, and indeed does it very well.

Its intended purpose is as a 'twat-ometer' - An approaching person is detected and the level of meter deflection shows just how much of a twat they are! Its supposed to be a simple joke device for workplace amusement. I won't include the circuit diagram here, as they are all over the 'net, and its such a simple circuit.

So, with regards to the 10m receivers, I intend to build an audio stage next as a stand-alone module, which can be tested on its own. That will then be used with the other circuits to test them out.

Saturday, 16 August 2014

How Do They Do It? - Or, why I cant buy local

I like to support local, and UK business, as much as the next fella. But, when it comes to electronic parts these days, unless its something very specialist, or I need it instantly, I can rarely justify it.

Take, for example, a recent need for 9-way D-type connectors and shells. To buy a pair of connectors and a pair of shells at the local Maplin store (ok, yes, I know they are dearer than the likes of Bitsbox and Bowood) would have cost me nearly a tenner. For around the cost of just one of the connectors, I instead have purchased 10x female, 10x male, and 10x shells, from the far east.

I just dont understand how they do it! All these are free shipping as well. Likewise, my stock of 2N3904 transistors is running low. A quid has bought me 100 of them from China. Even not allowing for the percentage of that that must be the shipping, thats a penny a throw. Same with opto's, a hundred mixed red, yellow, green, 3mm and 5mm LEDs from the far east - £1.10.

The same seller has an offer of 10x red laser diode modules for a tad over a quid! Packs of mixed size/colour heat shrink tubing for a couple of quid, a few hundred mixed value ceramic caps, or electrolytic caps, for a couple of quid. Its really quite easy to stock up on the basics for next to nothing from China, leaving you cash free to buy the more specialist parts locally.

Some detractors will say 'oh, but the parts are likely to be Chinese knock-offs!', maybe, for high value parts, but these are low value, mass produced, OEM components, and are, whether  you buy them direct or from a UK stockist, all made in the same Chinese factories anyway!

For such low value, high volume parts, it isnt cost effective for me to buy locally. I will still support UK companies for the higher value or more specialised bits, particularly G-QRP club sales, who have bits I can't easily find elsewhere, and are in fact almost as competitive on the small parts they stock as the Asians!

One part I cannot source form the far east, and will have to get from a UK seller, are the inductors for Daz's Theremin! Unfortunately, they are only stocked by Farnell Element 14, not exactly the lowest cost merchant!

What a Muppet

Yesterday evening, I tried to test my 10m Wispy on my Marconi 2955 test set, with shocking results. This morning, whilst I had a few moments spare, I fed the test signal direct into the RF amp, bypassing the filters, but had no better luck. I then fed it direct to the mixer, with still no luck.

It wasnt until a good while later that the niggle at the back of my mind (which sadly has had to contend with quite a few niggles these past weeks) resolved itself, whilst I was reviewing the details of the operation of the Polyakov mixer - This mixer has good AM rejection!

Bugger! Ive been testing with an on-frequency AM signal!

Part of the problem for me, I think, is that I tried an off-air receive test first, and didnt get a single received signal. But, propagation is poor on 10m at the moment, and although I looked at WSPRnet and noted not many signals, I didnt really correlate the lack of spotting by stations in central and northern UK with my lack of success, leading me subconsciously to think I had a fault.

A look back through Roger G3XBMs design notes, showed he achieved a -124dBm sensitivity without an RF amp, but with over 70dB of AM rejection, the AM only becoming detectable at -50dBm or so. Compare then with my, unaligned, -22dBm detection point of a 60% AM signal!

So, when I next get in the workshop, I will retest - this time using an actual carrier! Rather than set the test set to the actual tuned frequency of 28.1246MHz with a 1.5kHz modulated AM signal, I will instead just set the test set, with modulation switched off, to 28.1261MHz.

I do hope this is the only real issue. The RF amp is a grounded gate FET, and Roger used an MPF102. I didnt have any at the time of building, so substituted a J310 FET. This shouldnt be a problem. The only other change is to one audio coupling capacitor, and a bit of additional screening.

Last nights 472kHz reception was just as good as the previous session, best DX again 1141km into Germany, and again spotting Rogers tiny 5mW signal from over 190km from the small hours and into daylight, finally losing him around 06:00.

Friday, 15 August 2014

Poorly Wispy

Wispy isnt quite ready to be let loose on the wide world of 10m just yet, its a bit poorly.

The receiver is not working as expected. Detectable receive audio starts at an input signal level of around -20dBm, which is frankly rubbish. I would have hoped for -100dBm at least.

The fact it is so poor suggests something fundamental. When I next work on it, i'll check the transistors (ensuring their the right way round!) etc, but also will remove the coax link from the LPF to the RF amp, in case this is upsetting it. I'll also bypass the antenna switching diodes and see what receive is like direct to the receiver input. This is something im likely to do anyway, since I wish to use a small linear amp to give me a few watts of power, and relay switching will probably be the way to go for that.

So for tonight, all is packed up. I will monitor 472kHz overnight again. Already some decent spots are coming in, from Germany and the Netherlands.

Wispy On Test - Fingers Crossed!

My 10m 'Wispy' is now on receive test. It took a bit of time to work out usable level settings for the laptops microphone input, but im now showing the correct 0dB noise level, which in this case is a mic level of 50 and a 20dB 'Mic Boost' setting. Unlike when using a normal rig, where no mic gain is needed, the receiver here is very basic and the levels very low.

All I need now is an actual signal in band at a level strong enough to decode, or at least, see on the waterfall! But conditions have been very poor recently, with no spots even using commercial equipment, so this could be a long wait.

I also, have as yet not aligned the receive chain with a test set signal. Theres only two trimmers to adjust, but I dont want to tweek them yet. Save that for the Marconi 2955! For now, I'll just soak test the unit, check no smoke comes out anywhere.

In the meantime, I cannot use the main station for any HF work, since Wispy is on the main antenna! So, Ive moved back onto 6m to see what if anything might be doing in JT6M

UPDATE: Well, not a single spot received. I suspect that the band is pretty quiet anyway, which makes things harder when testing 'on-spec' like this.

However, I do know that the receiver works, at least after a fashion. The main station antenna is directly over the workshop. So, with the Wispy set up on receive in the shack, Ive set up my Marconi 2955 communications test set to give a signal on 28.1246MHz, with 1.5kHz modulation. Unfortunately, Im not lucky enough to own a test set that can produce SSB, so its a 45% modulation AM signal, and the maximum RF levelis 5dBm, into a telescopic whip. This is however just enough to detect as an AM envelope on the WSPR software!

Wispy Complete?


I have just finished building, to Roger G3XBM's design, the receive chain. This is a very simple direct conversion receiver, based around a f/2 Polyakov mixer, fed from the same 14MHz oscillator that drives the doubler for the transmit side.

The lower half of the board is the receive side. The mixer and RF amp is to the left, with the oscillator above. This is followed by an audio amplifier chain. The only change I have made to Rogers design, apart from the bit of coax from the Tx/Rx diode switch to the RF amp, is to use a 220nF cap to couple between audio stages, where Roger used a 470nF. This is simply due to the fact that I have no more capacitors higher in value than 220nF left!

I will rig her up for testing in a little while.