tuning receiver filters with Spec Analyzer instead of O-scope and sweep gen? - Page 1
Forum section: electronics / beginners

author: KD2FVM
author_rank: Contributor
timestamp: November 16, 2020, 03:04:39 pm
content: I have taken to repairing some older amateur radios for some friends, and find the service manuals are fairly complete with alignment instructions.Unfortunately they often require equipment I don't have like a sweep generator, or an AFVM.I've spent countless hours trying to understand exactly what they are trying to get me to measure (test points are provided) but I get stumped understanding what exactly I need to acquire both in signal and in additional equipment. I scoured the manuals of the test gear I have, but I don't see a direct explanation for what I'm trying to do.Is there a good source to read more about this stuff (without the requirement for a PhD in math)?I do have both an IFR1200, and a IFR 120B. I also have (or will have when it arrives) a Rigol SA815-tg, and a HP sig gen 8657a. I have an old Knight VTVM as well. I also have access to some high end DVM's from Agilent.

author: Paul Rose
author_rank: Regular Contributor
timestamp: November 16, 2020, 03:45:42 pm
content: With you new Rigol 815+tg, you would have to couple the TG output of your 815 into the input side of your radio's IF chain using a blocking capacitor, and also couple the output of the IF (again using a blocking capacitor) into the SA input of your 815.Your TG and SA output / input are both 50 ohm. Especially on the IF output to SA input you might want to use a very small value capacitor to reduce loading on the IF output. This will reduce the signal available to the SA, but the priority here is to not abnormally load the circuit being tested (which might distort the shape of the IF passband, which is the thing you really want to see).The actual "absoute" measurements on the vertical scale of your 815 won't be very meaningful in this setup, but you are looking for the shape.The 815 should be sensitive enough, (with its pre-amp if necessary) to make this work even though you will be losing some signal due to the coupling.Edit: deleted suggestion to use your 8657a external FM input to create a sweep using a function generator and oscilloscope in X-Y mode. It was incomplete.

author: KD2FVM
author_rank: Contributor
timestamp: November 23, 2020, 04:58:15 pm
content: I'm also trying to understand exactly what is meant by a AFVM. I understand that RFVM generally means using a RF probe on the DC input of a VTVM. I cant seem to find anything on the AFVM inputs though. Many of the radio receive section alignments suggest using one to read the actual output level referenced to 8 Ohms. I get the 8ohm dummy load but then what do I use to read for actual voltages? I've compared my O-scope, DVM, VTVM ac in, VTVM RF probe in, etc and seem to be all over the map. Got to the point of confusion of something that should be simple but I cant seem to find anything about it save for a few articles about vU meters.Any ideas where to educate my self on the subject?

author: TimFox
author_rank: Super Contributor
timestamp: November 23, 2020, 05:11:09 pm
content: Traditionally, an ACVM was one like an -hp- 400 series, a vacuum-tube device that displayed the equivalent RMS value from a measurement of the mean absolute value, assuming a sinusoidal waveform. The 400D had full-scale ranges from 1 mV to 300 V, and a frequency response from 10 Hz to 4 MHz, with an input impedance of 10 Megohm in parallel with 15 or 25 pF (depending on range).

author: Paul Rose
author_rank: Regular Contributor
timestamp: November 23, 2020, 06:54:56 pm
content: With a digital o-scope with measurement displays, and a 10x probe, you can probably see what you would have seen with an ACVM. It will be a bit more cumbersome.A modern DVM on AC scale could probably do it for post-detector audio. Check you DVM AC specs.

author: TimFox
author_rank: Super Contributor
timestamp: November 23, 2020, 07:13:01 pm
content: The traditional analog ACVM has lower resolution than digital measurements, but responds faster and offers a very sensitive (1 mV full-scale) measurement. The bandwidth is greater than almost all DVMs, and the update rate on a DSO measurement will be slower. Babies and bathwater...

author: Paul Rose
author_rank: Regular Contributor
timestamp: November 23, 2020, 07:19:46 pm
content: Quote from: TimFox on November 23, 2020, 07:13:01 pmYeah. I'm not suggesting to throw out an ACVM, only that you can use other available tools in a pinch.

author: TimFox
author_rank: Super Contributor
timestamp: November 23, 2020, 08:14:42 pm
content: The OP wanted to understand what the instructions for his older equipment meant by “AFVM”. I’m not sure if there any good ACVMs with analog meters still in production, but there are lots of the solid-state successors to the 400 series available used. One interesting possibility to measure the power into 8 ohms is the current version of the GenRad 1840-A output power meter, still available from IET for a measly $4150. It is a switchable impedance matching transformer into a power meter (2 mW to 20 W full scale). GenRad introduced the original 583 in 1932.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: November 24, 2020, 12:39:48 am
content: In most alignments of old radio equipment you are looking for a "max" somewhere.The audio freq volt meter is just that and measures audio output. You can just use an 8 ohm load or a speaker for the load and put the scope with a 10X probe across the speaker. Then just tune for max output while not disturbing your volume controls (RF and AF).On the IF stages you are adjusting coils (transformers) to let the IF through (and not other freq)Sweep generator is used for this but you can just tune the radio by putting in a signal into the antenna and then tuning the IF for Max.I did use an old signal generator for this in the past but a better thing to do would be to put in a signal modulated by 1 KHz at some point in the mid freq response of your radio. You can then tune the circuits for a Max at some measurement point. An example would be to monitor the output of the IF by using a scope after the IF and tune for max IF freq. There are probably rejection circuits and you can probably adjust those by keeping your radio at the set freq and moving the injected antenna signal up or down and adjust the proper coils for "rejection" you will be adjusting the IF freq bandwidth by doing this.You can also put a injected freq into the start of the IF chain and measure the output (again probably Max) with your scope, again for max and then vary the freq of the input signal a bit to make sure the "bandwidth" of the IF stage is OK.Does your IFR stuff have sweep? I would think they do....I am not familiar with IFR service monitors. I use HP service monitors and they do sweep.Wally