EMRFD Message Archive 12614
Message Date From Subject 12614 2016-04-07 17:06:59 bob_ledoux Seeking Receiver Diagnostic Techniques I’m building a 2N2-20, 20 meter superhet receiver. I’m seeking diagnostic methods to measure the performance of individual stages. My intent is to document the loss or gain for each stage as I move through the receiver.
My tools are,
Tek TDS1012 digital scope
Heathkit IG-1271 function generator
Homebrew AD9850 DDS RF generator
K5BCQ step attenuator (uncased)
Homebrew Norcal-type S-9 low level signal source
Chart of transistor pin voltages.
The signal sources have 50 ohm outputs. I’m using a standard 10X scope probe.
My approach has been to work from the speaker back to the antenna. At each stage I inject an appropriate signal and measure the increase or loss of signal as seen at the speaker. If I get signal clipping at the speaker I reduce the volume control and remeasure. The receiver has no AGC.
My injected signals are, 700Hz for stages from product detector to speaker; 11Mhz IF frequency signal from mixer to product detector; and 14Mhz from antenna to mixer input. The RF signals are tuned for product detector output tone.
This method looks good in theory, but I found inaccuracies. It appears better to measure the gain or loss between “closer in” stages.
I’d appreciate comments, concerns, limitations and alternative approaches to my question. Sources that layout procedures for the layman are appreciated.
12615 2016-04-07 18:42:12 kb1gmx Re: Seeking Receiver Diagnostic Techniques Though receivers to me are easiest as they will tell you if they are working.You hook up an antenna to it if you hear a noise increase likely its good.Signals you know are loud fro other radios should be. Weak ones youcan hear on another might even be better on the new one. Or not theredue to tuning or lack of sensitivity.Several things come to mind.Circuit loading, if a stated (input or output) are not properly loaded you will seeresults that may differ.Tuned circuits, a 10X scope probe will detune most if not all of them.This will lead to lower than expected in all cases. Tuned circuitsand devices connected to them must be handled with care.Hint the filter in that uses 11mhz crystals, likely the filter is centeredlower than that by some amount. So your DDS may be precisely at11mhz but the filter is likely at 10.998mhz For a CW filter that abig miss.Generally a chart of transistor voltages have proven mostly useless.If the device is good, a silicon device will have a base voltage that differsfrom emitter by about .65V. Why about? It varies with bias appliedand lot to lot variation. Now the emitter and collector are againvariable depending on the specific circuit. Oscillators may even stopwith a probe on it and the voltages will change!The tool I use the most is a 50ua meter with a pair of GE diodes on theback a cap (voltage doubler layout) across the meter and one is serieswith the diode input. I use it to see if there is RF present, any RF.Another useful device is a working receiver, even an old SX38 will do.Its best if it has an S meter for changes in level. Makes a good RFdetector, also gives you a hint as to the frequency (the scopescounter may see too much noise or not discriminate betweenseveral signals).A step attenuator with steps to 1DB. Use it as a substitution deviceto measure gain or loss.A piece of coax with a BNC plug and a loop (1-3 turns about .5" diameter)at the other end as a non contact RF sniffer for the scope.A antenna analyser or a resistive bridge and one of your RF sourcesas a tool for measuring impedance. Hint two ways to work thatuse the bridge in 50 ohm form to match a 50 ohm system or make theopposing side a variable resistor to find the balancing value and hencethe input resistance. The latter is handy when you need to make amatching network for an unknown to 50 ohms. Adding a Variable Cto that can make for useful impedance bridge for circuits with somereactance.Your ears, if you can start from the speaker and work backward to theantenna your ears will give a good accounting of gain or loss.Also what if an amp misses by 3db, is that the end of the earthor was it the dead stage bleeding RF through at a loss?I've built whole radios with just a frequency counter, DVM/VOM,the meter with the diodes (RF detector), a grid dipper as a RFsource, buzz injected using a screw driver tip, and my ears.When I first got in to ham all my test gear was for computerdevelopment. Things like multichannel logic analyzers areless than useful for a receiver project (ok now with SDR...).My first 6M RX was built by feel and still lives as its receiver isvery good. As needed simple things like a load with a diodedetector that can be made along the way. I have all the heavygear and access to more but most of the time they are turnedoff. Those are for the difficult cases or RF at UHF and higheror when I require precision. Its not so much what gear but howto use the gear to the fullest extent. That and understandingthe circuits in use.Allison 12617 2016-04-07 20:19:01 bob_ledoux Re: Seeking Receiver Diagnostic Techniques Thank you, Allison.
Allow me to describe my troubleshooting process.
The receiver is somewhat deaf. It should have an MDS of -128db. But a 10uv, 14Mhz signal at the antenna care be barely heard above the noise level. This receiver design is quiet in terns of background noise.
So my first approach was to forego a scope. Just listen. I started by feeding a 700Hz signal into the speaker, then audio amp, then the mute switch and then the preamp. The mute switch was conducting and I could follow the increase in volume as my injection points moved back. My later scope measurements suggested a 50db gain in audio stages with volume at max.
Next, I injected an 11Mhz signal into the mixer output. 11Mhz is also the IF frequency. I adjusted the signal source and local oscillator for maximum volume from the speaker. I could check the crystal filter for response by slightly altering the signal. I could tell the filter was sharp with good response curve.
Now, moving the injection from the product detector, back to the mixer, I could hear the increase from the IF amp, a small falloff from the four crystal filter and another increase from the post mixer amp. It appears the IF chain is doing its job.
Time for check of the RF states. A moderate level RF signal is fed into the antenna input. The VFO is tuned for audio in the speaker. The LC circuits for low pass and band pass filters are tuned for maximum volume.
Moving signal injection back towards the mixer I see a small loss (later measured as -4db) across the Norton RF amp. The amp is designed for a 9db gain so I'm missing 13db in the front end.
However, to get an audible signal out of the speaker my antenna input signal must be at least -90db. So I'm missing another 15db somewhere in the receiving chain.
I'm looking at the Norton amp. The transformer phasing appears correct. So I have not yet located that problem.
The mixer, or product detector, is a weak link in my diagnosis process. These are the bridges between one frequency and another. I haven't figured out how to measure the mixing loss which should be about -6db for the ADE-1 units. Because the signal gets through, I assume they work. Also note I haven't measured the VFO or local oscillator for a +7db signal for the ADE-1's.
the "just listen" approach I documented above works well. But I still have a 15db loss I haven't been able to locate just by listening.
This receiver is a proven circuit. Hundreds have been built. So I'm less concerned about measuring and matching impedances between stages. I appreciate your comment about the bridge circuit. It is more than "just an antenna tool."
I would appreciate your further comments as to how I can improve the process above.
I think we should encourage builders to learn basic troubleshooting skillss. There are too many projects that are built but never end up working. Builders haven't learned basic troubleshooting skills or are too proud to admit it didn't work.
12618 2016-04-08 08:41:27 kb1gmx Re: Seeking Receiver Diagnostic Techniques 2n2-20:Actually its an odd radio but a good design. Works well and fairly easy to build.it has a few unusual stages that require getting it assembled right to work. Also the VFOwith attention to parts used will net good stability I found a N150 for the base couplinghelped and splitting the 100PF to a 80pf COG and 20pf N750 and the 180pf as a COGmade it settle down to well under 100hz drift over time while using a 2n2222 there. Apreference and performance thing on my part.>>Moving signal injection back towards the mixer I see a small loss (later measured as -4db) across the Norton RF amp. The amp is designed for a 9db gain so I'm missing 13db in the front end.<<<
-13 db would be a substantial gain loss. You have found the offending stage.A 10uV sensitivity with +13db gain gets you in to the less than 1uv range or better.This is an example of the unique behavior of minimalist and gain limited radioswhere any gain missing has a notable impact on performance. Further its adouble hit. You loose the RF gain and also the net noise figure of the radiois degraded by the circuit loss. When its sorted out you will be impressedwith the performance.Suggestions...First check the transistor does not have the emitter and collector swapped. For that stage at 14 mhzI'd use a 2N2222 over a PN2222, I've not seen he same RF performance with the PN prefix partcompared to a real 2n2222.If that's not it build a copy of the post mixer amp on a small board (its a fine wide band design)and insert that in place of the Norton amp. Use that as a proof it can work. If it does workbetter then its a wiring or winding problem with the Norton amp. A common set of problem forthe transformer is more than one turn for the emitter feedback or swapping the collector for thepower end of the tapped winding. A voltage check of the transistor is Vcc on the collector,2V (or whatever the led voltage is) on the base and the emitter about .6-.7V less than the base.The voltage at the LED can vary and any voltage from 1.8(red led) to 2.8(blue led) the amplifierwill work (maybe some strong signal ability will change).HINT: Those DVB-T sticks with a upconverter board are very useful as a poor mans spectrumanalyzer and wide tuning receiver. If you respect the limited dynamic range (about 50db)its handy for measuring stage gains. That translates to do not use too much signal or too little.Allison
12621 2016-04-08 09:56:21 bob_ledoux Re: Seeking Receiver Diagnostic Techniques You've offered good ideas to solving this specific problem.
Could we go back to the general diagnosis procedure. My ear provides quantitative measure. I'm looking for expected gains or loss in volume due to the actions of individual circuit stages.
Is my process adequate in itself, or are there simple tweaks to improve it?
I have another question:
You made the following comment about the 2N2-20:"This is an example of the unique behavior of minimalist and gain limited radioswhere any gain missing has a notable impact on performance."
Is the 2N2 really "minimalist"other than being a single band rig? When I compare the stages to other production discrete radios of the late 1970's they appear similar. The 2N2 may not have some bells and whistles like a noise blanker, but it appears otherwise complete.
Can you suggest improvements that might make the 2N2 less minimalist and gain limited?
Thanks for your feedback. It has been very useful.
12622 2016-04-09 09:37:44 winston376 Re: Seeking Receiver Diagnostic Techniques Bob,Using the ADE and SBL +7 mixers required a strong signal level from my Vfo. A reduced Vfo level into those mixers showed symptoms as described in your post.Using an si570 based Vfo, my solution to increasing the power level AND provide 3 additional Vfo output ports was to use a 4 port HCPL ic isolator. I can simultaneously connect a freq counter, scope, SA as well as being connected to the mixer with minimal interaction. The HCPL provides a high impedance to the Vfo and doesn't, load it down at all. This architecture can provide an isolated second Vfo output for IQ Rx configurations...haven't tried that yet.As for your diagnostic procedure and equipment, it looks fine.Agree with Allison on her recommendations. I can only reinforce what she has stated and that is that standalone performance of individual stages will change when connected as a "system".Alex 12623 2016-04-09 09:47:35 winston376 Re: Seeking Receiver Diagnostic Techniques Bob,Forgot to mention....a method I'ved used for decades is to connect a vom or scope to the speaker while injecting a 400 hz modulated rf signal into the stage in question. Making adjustments or tuning can be easily "seen" using this method as opposed to just listening by ear alone.Alex 12624 2016-04-09 10:19:55 kb1gmx Re: Seeking Receiver Diagnostic Techniques How to improve troubleshooting... Experience and knowledge. Fix everything is sightas you will gain both experience and knowledge. Also try to build test gear and bitsto make you existing gear more useful to you. Understanding how things are measuredcan enhance the meaning of data your getting during testing. In all cases troubleshootingis problem solving and often the real key is getting to "What is the problem?".">>>This is an example of the unique behavior of minimalist and gain limited radioswhere any gain missing has a notable impact on performance."
Is the 2N2 really "minimalist"other than being a single band rig? When I compare the stages to other production discrete radios of the late 1970's they appear similar. The 2N2 may not have some bells and whistles like a noise blanker, but it appears otherwise complete.<<<<
You are confusing a description of design method and application with a qualitative description ofa radio. Neither mean its a bad radio if anythings its both clever and a good design for a very usefulradio. Also it was a application of language to describe function and application without technicalbias.Minimalist: The radio has been simplified to do an essential function and has only what is required.There is no noise blanker, no Notch filters, no variable IF gain, no variable selectivity, no auto-tuner,no digital display for frequency, no variable audio filter, and no AGC. None of those things arerequired, though they might be useful. A note, minimalist does not mean simple even thoughit can imply that.
Gain limited: There is enough gain to attain the needed level of sensitivity and noise figure butno more than required to maintain a useful dynamic range. The converse is so much gain thatthe dynamic range may be limited due to overload. Its a jugging act to get the signal out of thenoise for detection and avoiding added noise or overload that hampers detection. The questionmay be more gain to make up for what? Poor antennas may require more gain but then you arenoise limited for both antenna and environmental (galactic and man made noise). Always usethe best antenna, as the antenna is part of the receiving system (transmitting too).Neither suggest its a bad radio, they are descriptive terms on how its design was arrived at.Which if you looked into it it was really "contest radio", the contest being can you build a functioningradio with no ICs and not more than 22 2n2222 transistors? That places interesting constraints onthe designer and forces him to be a bit minimalist and creative as some of those devices are inthe transmitter. The fact we are talking about it over ten years or later makes its own point.FYI: there are many "contest" radio designs out there where the criteria was can you makea transceiver (or receiver or amplifier) for less than x amount of dollars (QST did that!) thatmeets a set of minimum criteria. The result is often very creative.For example even the most minimal radios like the Rockmite and clones have a Keyer chip.The keyer does not make the CW transceiver more powerful or improve the receiver. It doesmake it easier to send good CW. Now if the chip also controls a DDS/NCO/PLL then we improvethe radio some by making tuning less uncertain and more stable. However if done wrongit may add noise (lowering RX sensitivity) or lower battery life due to power needs increasing.>>>Can you suggest improvements that might make the 2N2 less minimalist and gain limited?
There seems to be the assumption that better is less minimal or more gain. That was neverimplied or said.As I see it as fully functional. If you increase gain you make break it and be forced to addcircuitry such as AGC to make it more complex. Adding more gain consumes more power.Doing so for a desktop radio is largely a non problem for a portable it could cost too muchin power used. Adding a SWR indicator might be an fairly passive add that adds utilityfor knowing power out and antenna performance. Same for adding a Auto or manual tunerto the basic radio. Same for a Keyer or an audio filter. Then again those can all be externalmodules that can be used for other radios.If you want to understand minimalist radios look at MicroR1, IQ-Crystal set, Pipsqueek,and many other that were examples of how few devices can you use and still have afunctional radio that meets some criteria. Also look at older radios like aTentec Argonaut 505 which can still be built using currently available parts. Its asomewhat minimal radio (in 1973 that was everything but 100W) by modern standards.BY 1973 standards it was 80-10 (WARC bands didn't exist for us) and 5W. It did haveRIT and it did have a SWR metering as well as an AGC and S meter. If it were a 100Wradio it would by then standards have most everything. Then you look at the high endrigs of the same era (Collins, Drake) and see what they added. By modern radio standardseven a minimal radio like the IC718 beats that. Its important to know the reference pointof view.Is a specification issue or is it that it works for you? For me if I find it fun to build or useits met specification.Hope that helps.Allison
12625 2016-04-09 10:38:30 kb1gmx Re: Seeking Receiver Diagnostic Techniques I really hate yahoo sometimes. It dumped a lot of typing...Troubleshooting... Its problem solving and thats both a skill and knowledge.Like one guy said he uses a meter to measure speaker output, calibrated ear.In the end its problem solving, first getting to the problem and then understandingwhat its needed to correct it. Building test gear can be helpful in understandingwhat the reading actually mean or can imply.>>>"This is an example of the unique behavior of minimalist and gain limited radioswhere any gain missing has a notable impact on performance."
Is the 2N2 really "minimalist"other than being a single band rig? When I compare the stages to other production discrete radios of the late 1970's they appear similar. The 2N2 may not have some bells and whistles like a noise blanker, but it appears otherwise complete.<<<
Minimalist is not the same a simple. Minimalist states there is no more than needed to accomplishthe task. Simple says its uncomplicated and easy to understand. Some minimal circuits are notsimple.Missing potential bells and whistles.. for the modern radio that includes:Keyer, audio filter, variable selectivity, SWR montitor, S meter, AGC, Passband tuning,automatic antenna tuning, and likely others that didn't jump to mind. Most all can beexternally added. Also many didn't exist for mid range radios of the 70s.>>>Can you suggest improvements that might make the 2N2 less minimalist and gain limited?<<<
The assumption is that's a negative thing. Its not. Its just language trying to be preciseas well as descriptive.Gain limited is a statement of design and implementation in the minimalist scheme.It has adequate gain to perform the function of good noise figure and adequate sensitivitywith a eye to overload performance measured as dynamic range.More gain can make a mess of things is not done right and listening to internal noisedoes not imply you can hear a weak signal. Enough gain is when the desired signal isdetectable with adequate signal to noise. The limiting factors are is there enough signalor does galactic and man made noise bury it at the weak end of the range. At the verystrong end can you detect it with out overload.In the end is the fun quotient met, if so then the specification is satisfied.Hope that helps.Allison