The Dummyloads

Where ham brains come to fry

Where ham brains come to fry

Archives for May 2018

Measurements and Learning

I have been disappointed to discover that my receiver is easily overloaded.  Also tuning the variable capacitor seems to have very little impact on the received frequency where as the adjustment on the front end seems to have a dramatic impact on the received frequency.  Why?  What measurements can I take and what can I learn?  Well I’ll tell you my story. . .

****Warning everything below this line is my best guess, I don’t actually know.  If you know I’d love to hear from you in the comments below.***

First of all I don’t have much equipment, but I do have an oscilloscope that Bill Ellis (N5TXN) gave me years ago.  It is a 15 Mhz BK Precision 1472C scope.  Unfortunately I don’t have any probes. . . Amazon 100Mhz scope probes ordered.  While I’m waiting on those, what else can I do?  First I used a dummy load to transmit 7.000 from another radio at 5W and used the variable capacitor on the front end to make sure I was peaking the signal in the CW end of the 40M band.  I then used the tuning capacitor to try to tune off the signal.  No luck.  Then I keyed down into the dummy load and started spinning the VFO.  I made it to 7.150 before the signal diminished in any way.  Hmm. . . that just doesn’t seem right.  Could my LO be on the wrong frequency?

How can I measure the frequency of the LO?

Many phone calls and emails to many friends.  A couple of notable quotes.

  1.  Tuning doesn’t matter if the receiver is overloaded.
  2.  You can hook up your oscilloscope or frequency counter, but know that you are loading the circuit and the frequency is not the exact frequency in the circuit.
  3. Given your LO arrangement, there just isn’t much signal to measure.

I was advised to use a 1″ form and wind 10 loops of wire around the 1″ form for a receive antenna.  I could then use this receive antenna to try to get a frequency count by using my MFJ-259B frequency counter OR use my oscilloscope and the antenna.  I was also advised that this was likely a futile effort as the total energy in the LO circuit is unlikely enough to get a good reading.  Turns out this was correct. . . The MFJ frequency counter gave me results from 400 Hz to 30Mhz and the oscilloscope could see a signal, but it wasn’t enough that I could actually count the waves.

I took a trip to the local Nortex QRP meeting and brought my little creation.  I expressed my frustration that the tuning capacitor seems to have no impact on the tuning.  After visiting with Dave Lear (NE5DL) and Joe Spencer (KK5NA) I became convinced that the issue was in my band spread capacitor.  It’s not a large enough tuning value.  When I arrived home my Oscilloscope probes had arrived!

I hooked up the oscilloscope probe to pin 6 on the NE602.  I got nothing.  I played with the RF gain pot.  Nothing.  I put my desk radio on 7.050 Mhz and keyed into a dummy load at 5W.  Hello. . . I have signal.  I guess that the LO runs at such a low current there is no observable signal until a threshold is met on the RF input.  I then spun my tuning cap and the frequency changed as did the amplitude.  The amplitude and frequency changed dramatically.  I interpret this as confirming my hypothesis.  The band spread capacitor needs to be adjusted to tighten up the tuning capacitor.  As the tuning capacitor exits the tuned portion of the front end the amplitude drops.  In my case, its REALLY fast!!

I think my next update will probably be finishing my final experiments with this receiver and start planning my next project.  We are approaching the time of year where operating outdoors is nice so I may take a break from building until it gets to be so hot that operating isn’t fun and field day isn’t far away either!

It’s ALIVE!!

A picture of the board with all of its hardware and a 9V battery for a size comparision.

In my last post I had everything wired and simply needed to add power.  Why on earth would I leave the devoted readers of this blog hanging on that moment?  Well I’ll tell you:  It didn’t work!

I said from the beginning I expected to fail, but learn through failure.  Mission Accomplished!

If you read my last post you probably caught my mistake.  I was mildly distracted while doing the final stages of the build with dad duty.  I also did not ever complete an entire circuit in one setting.  Even when getting power to the chips I failed to ground the IC ground pins.  In retrospect it really was a recipe for disaster!

When I plugged the power in I got a pop.  Good news, the LM386 is working correctly!  That is the extent of what happened.  I attempted to move the RF gain up and down.  I tried to “peak” the capacitor in the front end.  Nothing happened!  I started putting my screwdriver down on different pads on the board.  Pin 3 and 4 for the LM386 gave me a noticeable buzz.  Seems right. Pins 4 and 5 of the NE602 had the same result.  So far no concerns.  I put my screwdriver down on the antenna input, no change.  Seems strange.  I tried adjusting the RF gain, no change.  Hmmm. . .  Look at the picture, do you see the problem?  Well the truth is there were many problems, but the first was that I incorrectly connected the potentiometer for the RF gain.  I didn’t realize that a pot is actually a voltage divider network.  So your input goes to a resistor on pin 1, your signal feeds through at the voltage divide and you need a lead to ground for the second resistor.  You don’t just connect to the next step in the circuit.  Speaking of which. . . somehow in wiring all the front end I managed to put all the parts in, but failed to actually connect each of these components to each other to make a circuit.  DUH!  Even after adding a ground cable for the RF gain control and connecting all the steps for the front end, the receiver was still pretty deaf.  Hmmm . . .

I broke out the schematic and started going through each stage and mapping it to my board making sure that I had connected everything correctly.  Do you remember my drawing out the schematic.  If you look at the original schematic and my sketched schematic your going to see it.  Go ahead, compare pictures and tell me. . .

 

seriously

 

go

 

look

 

Pin 6 on the NE602 is the input of the local oscillator.  Pin 7 is the feedback.  I have them reversed.  Despite having tried to avoid the crossing of wires by sketching it all out, I have to cross pins 6 and 7 because of how I ran the capacitors.  The good news is that a hot soldering iron can help get your wires out and repositioned pretty quick.  I managed to do it with out permanently harming myself or anyone else.  For what its worth, it’s probably a good idea to wear glasses or safety glasses when hot solder and prying wire is involved.  Not that I found out first hand. . . but I’m not going to have cat-like reflexes for my whole life.  Ha!

OK. . . all pads connected properly.  Time to connect an antenna and give it a try.  What I experienced was magic.  Initially I was receiving local 96 FM station.  As I adjusted the input capacitor I lost the FM station and gained radio Havana Cuba.  They are very concerned about our President. . . me too I guess. . . prayed for the last one, will keep praying for this one.

Video of initial signals received

As I tweaked the capacitors I realized that I could here RTTY signals and the occasional CW signal.  True to the write up, the front end is pretty wide on this thing.  I think I sent an email to everyone I know to share with them my accomplishment.  It’s a below average receiver, but I did it!

So what now?

Well, I’d like to study what is actually happening in this receiver.  Take some measurements and attempt to optimize my LO tuning to be as selective as is possible with the capacitors I have.  Then I am likely to start another receiver.  Hopefully one that will be made entirely out of discrete components and also hopefully much more selective.  Maybe work on a filter of some kind.

More to come!