20-Meter High Performance Direct Conversion Receiver (6) with update

I did some troubleshooting of a motorboating problem with the 20m direct conversion RX we've been working on.  We moved the RX and TX circuit boards to separate cabinets in get more room around the boards and to cut possible interference between the RX and TX wiring, especially for the antenna tuner and oscillator variable capacitor wiring.  We also install and modification from the TX QST article (Dec 1978). The mod, shown in the diagram below, cuts the connection that supplies power to  the RX preamp Q3. The supply lead is routed to the transmitter "RX MUTE" switch, which supplies 12V to the preamp when the RX operates and disconnects the supply when the RX MUTE is active.

The circuit had severe motorboating, which went away when I removed the RX MUTE mod.  I think we can run without the RX MUTE mod,  so I'm going to leave it without the mod for now.  Later we can add an RF choke and bypass capacitance to the RX MUTE line that would probably fix the motoboating. 

The RX has a lot of hum and noise, but I suspect that's due to my setup:  I don't have a real antenna or even a decent ground in for my workbench.  

A bigger problem is that the VFO drifts too much to follow CW signals.  There are two capacitors in the oscillator tank that I will replace with NPO caps, to see if that cures the problem. 

Dec. 23: diagnose oscillator drift.  Replace C19 and C20 with NPO caps.  

• Drift is less severe
• Lowest frequency is about 14.2 MHz, need to fix

Dec. 23: noticed that oscillator produces about 500 mV RMS, but according to LTSPICE model and QST schematic, amplitude should be 2 V RMS.  Changed out the oscillator transistor Q4 for another MPF102

• Amplitude increased slightly to 600 mV RMS.  
• Disconnected oscillator from mixer by removing C22.  Oscillator amplitude increased to 800 mV RMS.
• Check 1N914 diode D1 forward voltage.  

20-Meter High Performance Direct Conversion Receiver (5)

 I simulated the single balanced mixer circuit is LTSPICE.  I wanted to see if the LO amplitude had a large effect on the mixer gain. 

• I was able to calculate models for the L1 to L5 input circuit toroids.  I lumped the tuning network into the gate drive transformer and found the tuning capacitance Ctune by trial and error. 
• According to the 2N2222 datasheet the small signal input impedance should be between 2Kohm a nd 8Kohm. I used 2 Kohm, shown as RQ3 in the circuit. 
• The audio transformer T1 is a Calectro D1-711.  I searched the Internet for a couple of hours trying to find the specs for it,  but eventually I gave up and pulled T1 off the board and measured its parameters with my LCR meter.  I was surprised that something the size of a small acorn could be 2.5 Henry, but that's what it was.  See the circuit for the parameters.  I measured the parameters at 1 kHz.  The transformer had significant series resistances, which are included in the model. 
• There is a 1:2 step up from the antenna across the gate driver transistor.  I modeled a 1 uV rms input signal and scaled it accordingly. 
• The LO is simulated at 14.000 Mhz, and the signal simulated at 14.001 MHz.  The baseband frequency is 1 kHz. 
• The conversion gain from 1 uV RF in to the baseband 1 kHz detected waveform across RQ3 is  12 dB.  Doug DeMaw, is his "QRP Notebook"  states that this kind of single balanced mixer usually gets about 10 dB of gain.  So 12 dB feels reasonable
• I simulated with LO amplitude of 2V rms, given in the magazine article, and 0.87 V rms as actually measured on the board.  There was only a very slight, barely noticeable drop in conversion gain. 

Base band output waveform is given below. 

20-Meter High Performance Direct Conversion Receiver (4)

Link to schematic

I simulated the oscillator in LTSPICE in order to get an idea of the expected voltage amplitudes.

The LTSPICE simulation schematic is shown below. 

The inductor is a tapped coil made from a T37-6 core. 12 turns for L1, 7 turns for L2.  The coils are coupled at unity using the K1... SPICE command line.  V1, the 9V power, is turned on using the 1 us ramp up.  I originally tried the simulation with a simple DC source for V1, but the initial solution wouldn't converge. 

After the transient oscillator startup, the plot of oscillator waveforms is shown below. 

The oscillator frequency is 14.88 MHz, so there is some stray capacitance in the actual circuit that isn't included in the simulation.  This stray capacitance would bring the frequency down into the 20 meter band.  

1. The top trace is Vtank, which is take at the C2.  The voltage is 16V pk-pk. 
2. The center trace shows the the JFET gate voltage.  This is about 13 volts pk-pk, and is limited at the high end by forward biasing of D1.  I haven't checked,  but D1 probably cuts in before the gate-source junction of J1, so D1 protect the transistor junction from being forward biased. The bottom voltage is limited by the cutoff voltage of the transistor. 
3. The bottom trace shows the  voltage at the JFET source, which is the output of the oscillator that is coupled into the mixer.  This voltage is 6V pk-pk, which works out to 2.1V rms. This rms voltage is given as 2V rms in the original article,  which indicates the simulation is realistic.  

As discussed in the previous post.  The actual voltage here in the unit I'm working on is 0.87 volts.   So the oscillator appears to be a sick little puppy.  I suspect that the JFET is damages or just a variant far from the normal characteristics for a MPS102...  The MPS102 is infamous for being wildly variable in its characteristics.  I will replace the transistor with a new one and it will probably improve. 

A cross plot of Id as a function of Vgs indicates gm is about 5 mmho for the simulated JFET.

I may try to figure a way to plot the drain characteristic of the actual transistor.  

20 meter High Performance Direct Conversion Receiver (3). Oscillator troubleshooting.

Link to schematic

Did some more diagnostic work on the this unit.  

Notes: 

1. The QST article says that the cathode of D2 should be at 9.1V.  Actual voltage is 8.9 V.  Good.
2. The QST article says that the C21 should be 2 V rms.  Actual is 0.87 V rms.  So the oscillator voltage is down significantly from what it should be.  I checked all the resistor values in the oscillator circuit and they are as expected. 
3. Removed C22 to see if the mixer was loading the oscillator somehow.  No evidence of that.  C22 was nominal 10 pF, and measured 9.6 pF.

There are other components to check in the oscillator circuit, but I suspect the transconductance, gm, of the Q4 is down and may be the source of the problem.  

I will order some MPF102 JFETs to see if replacing the transistor helps.  Perhaps the transistor was toasted while soldering. 

I will pull capacitor C19, C20, and C23 to check, and will pull L6 to check as well.

I am planning to do some simulations to check the dependence of the oscillator output on JFET gm. 

20-Meter High Performance Direct Conversion Receiver (2)

Partially checked out the RX ckt on the bench today. I powered the board off a 12V supply.   No external components were attached, including the VFO tuning cap and the peaking  cap.  The antenna coil is still missing. 

The voltage at  oscillator transistor Q4 drain is 8.9 volts, so voltage regulation of the oscillator power seems to be working. 

The oscillator, without the front panel tuning cap, operates at 14.365 MHz.    Amplitude is 638 mV pp, so 0.23 V rms.  

Next step will be to install a new antenna coil and repair the peaking variable capacitor.  I went to Four States QRP to order the toroids, but weren't in stock.  I ordered some T30-6 rather than T37-6, so I'll try to use those. 

20-Meter High Performance Direct Conversion Receiver (1)

Figure 1.  Photo 20m RX PCB

Today I'm starting work on the 20 Meter receiver that Mike bought back in 2018.  The receiver is based on a April 1978 QST article. (Page 11). 

• The circuit is fairly simple and shouldn't be too hard to fix.
• I found a 12 volt 2 amp "wall-wart" power supply to power it with.  I don't have a jack to go with it.  I'll have to kludge something.
• The toroid that comprises L1/L2/L3 is missing.  I may have to order a T37-6 core to replace it.   

 

Layout of the reciever board taken from the magazine article. 

The view is from the copper side.  The component side is a mirror image. 

20 Meter High Performance Direct Conversion Receiver & VXO Controlled 6W Transmitter

Well, I haven't been posting for a while.  My QRO rig (Yaesu FT-101ZD) has developed a problem with the VFO.  The frequency jumps around.  I have no idea what could be wrong.  My only guess is that there is a loose connection.  I suppose if it were a capacitor it would drift instead of jump.  I plan on at least pulling the board and have a look-see.  I wonder if I could get the external VFO and just bypass the internal one.

I have bought a You Kits HB-1B qrp rig from Ten Tec and am quite pleased with it.  I have made QSO all around the world with its spankin' 5W.  Last night I made contact with a station in New Zealand and received a good report.  I took it camping and had a ball operating from a park bench next to the lake.  The ticks and chiggers were there to great me....never going back there again.

My other QRP rig is the Rockmite 20 and it is a little harder to make good QSOs but I have worked coast to coast with it and yesterday morning I held a 1 hour rag chew with a fellow in Canada camping out in the boonies.

20 Meter High Performance Direct Conversion Receiver & VXO Controlled 6W Transmitter

Look in QST Apr & Dec 1978 for the article on how to build this transceiver.

I purchased this homebrew QRP rig off of ebay from its builder K1NYK.  The workmanship is very nice and I hope to have it up and running soon. Taken from the 1978 QST article it uses a VXO on the 20M band and has a direct conversion receiver (the lower PCB).  It should put out about 6-7W.  I tried the receiver but all I get is static.  This will be my first attempt at repairing a radio.  I first try to find the problem with a voltmeter.  I don't have a signal generator or rf detector and I guess I'll have to make something from plans off the internet.

I finally have accumulated all the parts for building a replica of my brother's QRP rig he designed that are posted elsewhere on this blog.  The NE602s where not that hard to find.  I've already started the project with a proto board.  I better get her done before I lose some of the parts. :)

UPDATE: July 25, 2014  A fellow ham has suggested that I clean the VFO capacitor to try and resolve the freq stability problem.  I've built and audio oscillator using a 555 timer to test the receiver and the circuit from the mixer to the phones is good.  Also I tested the transmitter and it is also good.  So I suspect the circuit leading up and including the mixer and BFO.  I'm trying to find out how to test those circuits.  I wish I had a signal generator that would do AF and RF.