Tuning a transciever with a SA

[large_ghostman]

I would like to adjust a CB radio and the transmit frequency a little bit using the spectrum analyzer (unless my FC proves to be accurate), what do I need to sample the transmit frequency? I am looking for something to go from the transmitter to a device (the bit I dont know) then out to a dummy load, I believe there is a device with a BNC take off that goes to the SA but I am completely unsure what you use or how you connect it. Sorry if this make no sense! I am wanting to adjust a CB whats the best way to do it with a SA and dummy load?

 

[JimB]

I would like to adjust a CB radio and the transmit frequency a little bit

How much is "a little bit"?

using the spectrum analyzer (unless my FC proves to be accurate),

You measure frequency with a frequency counter, if the counter isn't accurate it is no good.
A spectrum analyser measures other stuff.

what do I need to sample the transmit frequency? I am looking for something to go from the transmitter to a device (the bit I dont know) then out to a dummy load, I believe there is a device with a BNC take off that goes to the SA but I am completely unsure what you use or how you connect it. Sorry if this make no sense!

A 20dB or 30dB Coupler is a good start. You may need some more attenuation between the coupler and the counter and or analyser.

I am wanting to adjust a CB whats the best way to do it with a SA and dummy load?

Define "adjust"

JimB

 

[jpanhalt]

An added comment: When aligning my RC stuff, we never use a copper-to-copper connection, It is always RF-to-nearby antenna.

John

 

[large_ghostman]

How much is "a little bit"?


You measure frequency with a frequency counter, if the counter isn't accurate it is no good.
A spectrum analyser measures other stuff.


A 20dB or 30dB Coupler is a good start. You may need some more attenuation between the coupler and the counter and or analyser.


Define "adjust"

JimB

I will try and find a link to show you what I am doing, BUT no shouting!

 

[large_ghostman]

**broken link removed**

I am adjusting this for 10M using the method as described, I havnt attempted anything like this and my RF sucks as you know .

I am more concerned that it will transmit on the legal frequencies after the modifications than I am about tuning for the extra channels, however as i hope to sit my exam in April (first one) I am trying to mod it now. The club i am going to do the exam is Stranraer

 

[JimB]

When aligning my RC stuff, we never use a copper-to-copper connection, It is always RF-to-nearby antenna.

ALL transmitter tests/tuning/modifications should be done into a non-radiating dummy load. Your RF neighbours will be very annoyed if you do not.

Only when the transmitter is operating correctly should it be connected and matched to the antenna, radiating RF.

JimB

 

[large_ghostman]

I did say dummy load honest! I wouldnt use an antenna until I was sure it was ok, hence why I was asking the question

 

[JimB]

I did say dummy load honest!

Yes, I know you did.
I was replying to Johns comment.
Put your new glasses on LG.

JimB

 

[large_ghostman]

Yes, I know you did.
I was replying to Johns comment.
Put your new glasses on LG.

JimB

How did you know I didnt have them on? .

 

[jpanhalt]

Yes, I know you did.
I was replying to Johns comment.
Put your new glasses on LG.

JimB

It was a certified repair station. The transmitters were previously certified. Had them re-checked about every 2 years. I assumed the technician was doing it correctly.
John

 

[JimB]

John
That is OK for a quick test of a presumed working low power transmitter which I guess does not have an easy obvious connection for a dummy load.
However, for modifying a radio or building one from scratch, the initial testing should be done into a non-radiating dummy load.
Having said that, some dummy loads do radiate quite well, which can be embarrassing sometimes.

JimB

 

[jpanhalt]

John
That is OK for a quick test of a presumed working low power transmitter which I guess does not have an easy obvious connection for a dummy load.

That is what was being done -- just adjusting for crystal aging. The transmitters do not typically have a place for a dummy load.

However, for modifying a radio or building one from scratch, the initial testing should be done into a non-radiating dummy load.

Agreed. A good friend in school was an active general license ham. He didn't have a spectrum analyzer (SA), but still used a dummy load when adjusting his transmitter. The power difference between his XMTR and my RC stuff was quite different. I did not mean to imply that a powerful transmitter should be adjusted while on the air.
In the US, building or modifying a CB still requires a license, I believe. My concern was that spectrum analyzers are typically fairly expensive relative to CB radios. A direct copper to copper connection could well damage the SA, unless one were sure before doing it or relied on the weak and unintentional radiated power. Maybe more expensive SA's have protection built in, but there have been some comments posted on forums about the potential to damage the inexpensive ($1500) Rigol SA with overload.

John

 

[JimB]

A direct copper to copper connection could well damage the SA, unless one were sure before doing it or relied on the weak and unintentional radiated power. Maybe more expensive SA's have protection built in, but there have been some comments posted on forums about the potential to damage the inexpensive ($1500) Rigol SA with overload.

Correct, connecting anything which outputs more than a few 10s of mW will fry the input circuits of a spectrum analyser.

Say we were testing a transmitter which had an output of 100watts.

We would connect the transmitter to a dummy load via a 30dB coupler.
The coupler will create a tap and direct 100mW of power towards the test equipment.
100mW is still a bit much, so maybe put a 20dB attenuator between the coupler and the test equipment.
We will now have 1mW to feed into our spectrum analyser, frequency counter, modulation meter, oscilloscope or w.h.y.

Connect it up like this:

(No, that radio will not produce 100Watts!)

The insides of the 30dB coupler:

A couple of 20dB in-line attenuators:

And a couple of dummy loads.
A Bird "Termaline" coaxial resistor as they call it, rated at 50 Ohms 50 Watts, and a home made one which is fan cooled rated at 50 Ohms and 100 Watts (ish).

JimB

 

[large_ghostman]

Thanks Jim I didnt know what I needed to get! So a attenuator and a coupler, I feel a bit cheated seeing inside a coupler! I was going to make a Dummy load and I am tempted to make a coupler if I cant find one a reasonable price on ebay. I have no idea what these radios put out but its way below 100W, both have been opened and messed with but I would think the output side is the same as it was. They were cheap so ideal to practice with and I now have somewhere to put them, thanks again Jim that is exactly the info I was looking for .

 

[large_ghostman]

One last daft question (for now) I take it the Attenuator also blocks DC?

 

[JimB]

The attenuator will attenuate DC, it does not block it.
There should be no DC on the output of a radio transmitter.

The coupler is easy to make, you need:
A small ferrite toroid (of the correct mix)
A suitable box.
Three coax connectors, I use BNC.
A short length of coax for the through line.
Wire for the winding on the toroid. 10 turns for a 20dB coupler and 31 turns (from bad memory) for a 30dB coupler.

JimB

 

[large_ghostman]

There is a video on youtube on how not to kill your spectrum analyzer, I got the DC block from there its made me a bit paranoid considering how long it took to get a spectrum analyzer. On toroids what mix am I looking for? I looked on ebay and couplers dont seem that common so making one is the way I will go, I take it a Aluminum box will be ok? Does it matter on the gauge of wire I use for the torroid? I have found some Attenuators that will fit the SA they are a bit pricey but not as pricey as a blown front end would be

 

[JimB]

Toroids
I used FT37-61. The 37 is the diameter (0.37") and the mix is 61.

Box
Aluminium or a diecast box is fine.
If you are feeling lucky, you could build one from double sided circuit board material.

Attenuators
Are also quite easy to make, only three resistors. But you have to have the right resistors. Sometimes it is necessary to parallel up some standard values to get the correct value.

Wire
Not too critical, about 26SWG is a good starting point.

JimB

 

[large_ghostman]

Thanks Jim, I remember the conversation about SMD resistors etc, for what they cost and because of the connector I might just buy the Attenuator at some point. I will have a search around for toroids. I only use single sided PCB but I have a metal box and loads of tins somewhere . Dummy load I might make oil submersed as I have alot of mineral oil sitting around.

I might even try and get the other radio working, it's shorted somewhere and draws around 6A when you plug it in.