Signal generator to receiver coupling-How?

[djsbriscoe]

Hi,
What is the best way of coupling an RF signal generator(in my case, a Marconi 2022E) to a radio receiver?
Sometimes the service manual specifies that the generator is connected to a specific test point. How can this be done without loading the circuit unnecessarily? What is the best way of connecting a generator to the antenna input? How can an ACCURATE signal level be injected to test for receiver sensitivity?Hope someone can give me some advice. Thanks.

[chriswood1900]

No knowing what radio you are connecting to the advice can only be general but this should help:-
https://www.vintage-radio.com/repair...alignment.html
4th Sentence under AM alignment.

[Bazz4CQJ]

I think a lot depends on the receiver; is it a fairly run of the mill broadcast set or does it have even the vaguest aspirations to do something better. Attached is a circuit which has been proposed to simulate the effect of having the Rx loaded on to a long wire aerial while injecting the sig gen.

Ideally, you'd hope the service manual would advise.
B

[Sideband]

The Advance signal generator I use has a 'dummy load'. Some service information recommends the use of a dummy load, others just advise 'Loose coupling' via a loop, the dimensions of which are given in the service information. Some advise just coupling via a capacitor at specific points again advising the value of the capacitor. Some service information gives details of suitable dummy loads to make up. As regards the correct input, the general advice is to turn the AF volume to maximum, connect a millivolt meter across the speaker, inject the signal at specified points whilst monitoring the reading on the voltmeter which must not exceed a certain voltage (usually 50mV) to ensure that the output of the signal generator is not excessive. It's only if you are aligning a good quality communications receiver that you really need to get the signal levels super accurate. Domestic receivers are far less fussy.

It all sounds more complex than it really is but the alignment instructions will usually advise where and how to inject the signal.

[Skywave]

If you're not interested in sensitivity measurements etc., (e.g. doing a full re-alignment) then almost any form of 'loose coupling' will do to inject the test signal to almost any point in the receiver, using a suitable d.c. blocking capacitor. However, if the radio has a stated aerial input impedance of 75 ohms and you do wish to make sensitivity measurements etc., you will need a 50 ohm to 75 ohm matching pad with a known loss.

Al.

[Cruisin Marine]

There was some fairly extensive guidance on this for earlier Marconi generators, it was either in the 2015 or 2008 manual, can't remember exactly.
The manuals for both are pretty easy to find for download though from memory.

[Cruisin Marine]

I just remembered, the coupling info and impedance matching info. was in the Marconi TF2002 manual.

[ms660]

Advance TP1 (75 ohm) dummy antenna termination pad here (link in Post#1):

https://www.vintage-radio.net/forum/...ad.php?t=80264

Lawrence.

[Bazz4CQJ]

Originally Posted by ms660

Advance TP1 (75 ohm) dummy antenna termination pad here (link in Post#1):

https://www.vintage-radio.net/forum/...ad.php?t=80264

Lawrence.

Interesting article, but it's news to me about things working from 10 R impedance. Was this a standard for old broadcast band receivers?

B

[ms660]

Originally Posted by Bazz4CQJ

Interesting article, but it's news to me about things working from 10 R impedance. Was this a standard for old broadcast band receivers

Apparently so, a dummy antenna is mentioned in a lot of service manuals.

Lawrence.

[ms660]

An output impedance of 10 Ohms or thereabout was quite common in some of the earlier signal generators, eg: Marconi TF144G.

Lawrence.

[djsbriscoe]

I am considering aligning a Revox B261 FM tuner, and I have a few NAD 4300 tuners to have a go at as well later on. I've attached the alignment details for the Revox below. Just wondering what's the best way of coupling my Marconi 2022E to it?