DC: Polyakov (I)

@Oct/2 2015

Full schematic at very last section on this page.

microR1

I ever built some junk Superheterodyne, not a serious receiver for amateu .  <<EMRFD>>  suggest micro R1,  a simple DC receiver,   it's a good start point.   Here is the micro R1 from <<EMRFD>>.

I actually not  prepare to build this receiver.  The T50-2 is expensive compare to the junk surplus IF transformer, I happen to  have hundreds of IFT.  The 7.040/7.050 or 7.020 crystal resonant is not available yet for me.   I'm on a vacation, huge festival, shop closed.  And the output audio transformer is also not available for me.

 AA1JJ Polyakov DC

There is an interesting mixer,  introduce by RA3AAE,  Vladimir Polyakov.   Michael, AA1TJ, is a big fan of this circuit.   I's known as  "Russian mixer",  AA1tJ  present a 40 meter DC receiver  Polyakovs russian mixer

(Original link missing, here is another Russian site:  Mixer on 'Radio' Magazine) 

The fancy part of this mixer is the LO only need to running at half of the 7.050 Mhz!  i like VFO design, but VFO drift, i never try a VFO on SSB reception(mostly because i don't built the SSB receiver -:) . 

This receiver's font end use FM IF CAN,  I have bunch of FM IFT.  Schematic suggest  4 uH IFT,  but a different value is also works. 

I gonna make a  receiver base on this one.  Keep the DTC front end,  LO,  audio diplexer,  replace the audio stage with LM386, which could support SSB. 

Build the First DC experimental receiver

Build the Audio stage first, LM386 with 100X voltage gain. Then the pre-selector, and bang, The prototype were done!

First Running

OK, need a Antenna, built one. 

I might make  tons mistake.  The vertical antenna 5 meter high,  10 Meter wire wound  on 3 meters rod. Don't  know what frequency it resonant on.  Receive only antenna, random end feed antenna might be better than this.

On an afternoon, turn it on,  loud hiss come from it,  the station sound is so light to be heard.  I suspect LM386(40dB)  won't have enough gain  compare to AA1TJ's audio stage.  But, finally signal  get stronger, I heard SSB !  but with new built audio stage which provide tons of gain. 

The simple R1 or any DC receiver with low gain audio stage,  need a good antenna,  and  waiting some one xmit strong signal.

Fight the Gain VS motor boating

i read the gain request for a receiver from somewhere, forget, maybe JF10ZL, at least 100 dB gain required by a receiver,most of time. obviously, i built a DC receiver don't have enough gain, plus the detector have no gain but loss. so, i considerate adding a audio amplifier stage before LM386.

Todd, VE7BPO running the really awesome site: http://qrp-popcorn.blogspot.com,  who fans  W7ZOI's audio amplifier from the "ugly weekender transceiver".  based Todd's experimental, this is a low noise high gain amplifier, definitely worthy to try.

the left one is VE7BPO's audio pre-amplifier, the right one is redraw with consideration the layout method, i do fans this style schematic 

c1-c3:  CW 0.047uF, SSB 0.015uF

I prefer put transistor on top of the schematic,  most old schematic and Japanese guy fancy this way.  I personally think this is clear way to emphasis the AC path of a receiver.  The DC bias then not make your mind mass.  Note, my version of this circuits omit the Q10, the follower.

the following photo show the picture of this audio stage.  note, the right one have a very long twist line feed the signal to LM386 Audio PA stage.

you can imagine, how bad the layout is. motor boating?  I don't want to change my bad layout(too hard), instead,  I lower the gain by replacing the R34 with a  4.7k resistor, then motor boating stop. 

There is another way to stop motor boating and get more gain, extract signal from first 2N3904's collector,and then increase LM386's Gain.  JF10ZL preset a way to get 70dB gain from LM386, that's really noisy, but works(my choice, the 54dB gain). 

DC receiver, performance and problem

problem:

•  VFO: drift, can not stay on SSB beating for more than 1 minute, why? bad shield, no buffer, no tune for stability at all. soon or later, i will try Vacker.

•  broadcasting pickup: day time, no, because shortwave station closed -:), evening acceptable with the DTC.

•  micro-phonics, it does, but this construction is not that Mechanical stable, it's for experiment.

•  tune rate:  comfortable, beating SSB not that hard. you might notice i use a very good gear driver for reduction about 30:1, 200pF air capacitor.

about the mixer:

     I tried 2 type diode for this, 1N4148 and 1N60, which is better?  might the 1N4148, 1N60 will severely loading the oscillator. the 1N4148 sound a little bit quite, but can not confirm this from one time experiment.

Get a Oscillator which finally stabilized at room temperature

NOTE: stability observed here is the result  after power on the oscillator about 10-15 minute.  temperature stability is not under consideration.

i do not want try to make oscillator  stability from one temperature to another.  the oscillator i built here, just a bunch of crap,  the frequency jump from one to another frequency point quickly.  one most of annoying phenomenon of the oscillator, is the frequency jump, i mean, this second, get 7.060 000, next second to 7.060 023,  next to 7.060 059,  7.060 010,  7.059 980, 7.059813 ... jump , jump, jump and at last, far away from the 7.060 000. seem never stable at one certain HZ.

the Monolithic cap(X7R etc..) version performance, the first version:

         *  can not hold SSB signal for more than 1 minute.

         *  Jump Hundreds of HZ per second, up and down, seems cannot stable at certain HZ

         *  drift quickly from choose  frequency, trend is to higher frequency/or lower frequency, very quickly

         *  definitely can not be same HZ in adjacency second period

Let see what i have done to the oscillator. I do read the VFO built guide from <<EMRFD>> <<RSGB>> or many many other guide. but i never flowing one of them.... -:(

Let me try first thing, use high stability capacitor. i DO use the NP0 for C11, and VC1 is the variable air capacitor.  the most bad capacitor i used is the C12, 2200pF,  i solder a  monolithic chip capacitors, the middle yellow one in the photo. 

the Monolithic cap is the terrible capacitor for oscillator, crap of performance.   first trying is replace with Mica(not it is silver Mica), the brown one in the above photo. let's directly list the performance points.

the polystyrene capacitor is one of common cap for compensate VFO drift. let try, the the photo, actually this is a acceptable stable version.

does this useful? a little bit, definitely need totally shield for temperature isolation.

the Shield Polystyrene version performance:

         *  could  hold SSB signal for a long time, better but not that obviously.

         *  Jump 10-20 of HZ per second at warm up period

         *  could stay at  same  HZ in adjacency second for few seconds

         *  Room temperature Long time test: after 8 hours, it still around the choose frequency,+-100Hz-200HZ

Simple Shield version Mica(silver mica) update(next day data): 

         *  could  hold SSB signal for more more than dozens minutes after totally warm up.

         *  drift UP from cold on,  after 10-15 minute, begin to stable:  frequency  jump slow down to few HZ/second

         *  totally warm up need about one hour  or longer

         *  could stay at  same HZ in adjacency  second for seconds after warm up

         *  after warm up,  one hour could drift 150 HZ (mostly UP)  

some hints,  not anything is new:

     * use good capacitor

     * Shield ,shield, shied:  eliminate frequency jump, i thing because it shield Electromagnetic interference and keep temperature stable.

Get Oscillator warm 'down'

@Oct 15, 2015

all most all my capacitor is small negative TC capacitor, and unfortunately it's drift up when warm up. i hear the poly capacitor is positive TC, it's worthy to try, even though there is no suggestion to use them in the oscillator.

c12=> 2.2nF Poly cap: 

•     could hold SSB signal for more more than dozens minutes after totally warm up.

•    still drift UP from cold on

•     stable time reduced, 10-20 minute is enough, and drift about 7 kHz - 9khz

•     could stay at same HZ in adjacency second for seconds after warm up

all most all my capacitor is small negative TC capacitor, and unfortunately it's drift up when warm up. i hear the poly capacitor is positive TC, it's worthy to try, even though there is no suggestion to use them in the oscillator.

c12=> 2.2nF Poly cap: 

    * could hold SSB signal for more more than dozens minutes after totally warm up.

    * still drift UP from cold on

    * stable time reduced, 10-20 minute is enough, and drift about 7 kHz - 9khz

    * could stay at same HZ in adjacency second for seconds after warm up

OK, my C11 is parallel many small capacitors, i highly suspect these small capacitor might tend to be small Negative TC capacitors(one capacitor have a Red point, means N075 ), capacitor check by my multi-meter also confirm this.  the Mica capacitor shown in picture seems have very small  TC, so i decide to replace C11 with Mica, and C12 with the 2.2nF poly capacitor.  finally get a warm "down" oscillator, then it's easy to compassionate to be a stable oscillator.

C12=>2.2nF Poly cap,  C11:470pF Mica( confirm it's silver mica): 

    * Finally  drift down from cold on

    * 10 minute later frequency jump reduce to few HZ/seconds, and totally drift about 3kHz

    *  totally warm up need about 30 minutes, even more

    * could hold SSB signal for more 30 minutes after totally warm up, and drift 200Hz in an hour in room

C12=>2.2nF Silver  Mica,  C11:470pF Silver Mica: 

    *  drift down from cold on

    * 10 minute  frequency jump reduce to few HZ/seconds, and totally drift about 2khz 

    *  totally warm up need about 20-30 minutes even more

    *  could hold SSB signal for more than 30 minutes after totally warm up, and drift  Hundreds of HZ in an hour in room

how to get a oscillator compensated?

cause i have some silver mica, some Polystyrene and N750 capacitor. (plus bunch of crap NP0, which mark as NP0 but most of them are negative TC ceramic capacitors.)  also some poly capacitors.

most of my torrid and oscillator coil are Positive TC, if a initial oscillator are warm down, the positive TC, things became easy, replace some capacitor with negative TC capacitors which easy to get. 

so, my idea is use all capacitor with my silver Mica, most of time it's positive TC. then replace one capacitor with combine SM+N750(or polystyrene or  poly).

performance:  

*  warm up time about 30 minutes( NO shield)

*  could stay at one frequency(i.e listen to SSB ), for few minutes to half hour,  don't not need to re-tune.(room)

it's not  compensated enough but still get improve, the shield should give more performance improved, it's ensure temperature stable and electronic isolation.

Stop moto boating

first time to build so high gain audio amplifier, worry it would "boating", it did, sadly.  my circuits based on VE7BPO's DC frame, it does tend to motor boating until i replace the 1k resistor with a 4.7k one, actually low the stage gain.

I refer to QST "ugly weekend" original version , this audio pre-amplifer,  have a diffrent with VE7BPO's audio stage. the 1k resistor connected to Q11's (following schematic) emitter not the VCC, it definitely a strong decouple worth to try. 

finally, it's stable.  i redraw the circuits, compared to original, omit one output buffer, cause this stage will drive the lm386, very high input impedance.

another interesting audio preamplifier come from  KK7B High performance DC receiver's audio pre-amplifier.

Use the LM386

even thouth it's noise, it's still a quick audio PA for small project.  for quick project, the dead bug layout is prefer. to simplify my next project. here is the layout.  

several days listening i feel frustrate about the drift problem,  it's warm up drift 2-3khz, even more.  it's sensitive to temperature changing too much.  and warm time it's too long. 

i finally decide to shield the VFO, and replace the VFO tank inductor  with a nylon bolt, i saw this method  introduce by BITx transceiver, and it's attractive to me. 

by changing the T3 to nylon and shield, this version is far more stable than previous version. 

•  warm up time reduced

•  warm drift (down) about 500Hz to 900Hz (depend the room temperature, it's Winter here now)

•  after totally warm up,  several hours listening to SSB is possible,  without  retuning.

•  at morning and sunset, temperature changed much, and this VFO will drift more than daylight or night, need retune. 

almost all the capacitor i used here is negative TC,  but the VFO still drift down after the temperature rise.  via heat the parts one by one, it's show that the most sensitive part, the Q3,  a 2N3906, is the most POSITIVE TC parts.  it's almost could not make the VFO  TC get positive by using compensate capacitor.

ok, i think i got something really valuable experience to build next VFO. so i stopped here, and stop building this DC receiver more.

One Day experience and violence compensate Process

Nov/16

In summary，temperature changing within 200 Hz in morning and change 500 Hz in whole day. while temperature come to same, the frequency will come back too. IT's need retune many times, even in restroom.

And warm up drip about 500 Hz within 30 minutes.

Obviously it's negative temperature coefficient.  and I need a effective method to metering the temperature coefficient to speed up compassionate process. I got a violence wild idea.

blow the circuits from 30 cm away, aster 10 minutes you will got stable frequency(+- 20 Hz in one minute). So, i got 2 stable frequency on 2 temperate. based on this, change the compassionate scheme could be achieved within 30 minute. 

scheme One:

c11: 220 N450, 470 mica,

c12: 2000 mica

30 degree to 65 degree: frequency dropped 9 khz

scheme  Two:

C11: N470 220 pf X 3

C12: Mica 2000 pF

frequency increase  3 kHz

Scheme Three

C11: N470 220 x 2+  250 pF Mica

C12: Mica 2000

Frequency increase 2Khz. Take this one. 

By take Scheme Three, the actuality normal use in room temperate with free open or close the window, frequency changes less than +-50Hz in entire day.  Warm time is fairly quick, less than 5 minute and drift around 100 Hz. 

Final note

@02 Nov, 2015

 *  SSB need stable VFO, or use crystal

*  audio stage have huge gain, should well decoupled.

*  Polyakov mixer is simple and amazing simple

*  Get yourself a long wire antenna if you don't have