40m DDS Transceiver Project - Spectrum Analyser - What am I seeing?

[SwingeyP]

Hello again,

I am still battling with my 40m transceiver project and along the way have acquired a TTI TSA1000 spectrum analyser adapter.
I have to admit that i'm not entirely sure what I am seeing when using it.

A couple of questions :

1) Does anyone have a copy of a manual for the TTI TSA 1000 adapter they would send me?
2) With no input connected to the adapter I see a peak at approx 5mhz - Why?
3) If I look at the output from my DDS (running at 3.2 mhz) I see 3 (main) peaks 2, 5, 9 - makes no sense at all. I was expecting to see a peak at 3.2 mhz can anyone explain this please?

4) When I eventually get that far how do I use the analyser to see the response of the xtal filter I have just made? - It appears to be doing something as on the scope I now see a much nicer single sine wave rather than that shown in my previous post straight out of the mixer.

A little knowledge would be dangerous but as I have none I am a liability

Thanks in advance

Regards - Paul

 

[JimB]

1) Does anyone have a copy of a manual for the TTI TSA 1000 adapter they would send me?

Sorry, no.

2) With no input connected to the adapter I see a peak at approx 5mhz - Why?

That sounds like the "zero frequency" marker.
I am not familiar with that analyser, but it should be possible to tune the analyser to put that marker at the left hand side of the screen.

3) If I look at the output from my DDS (running at 3.2 mhz) I see 3 (main) peaks 2, 5, 9 - makes no sense at all. I was expecting to see a peak at 3.2 mhz can anyone explain this please?

Yes.
Zero is 5Mhz
5Mhz + 3.2Mhz = 8.2Mhz almost 9Mhz with dodgy calibration.

5Mhz - 3.2Mhz = 1.8Mhz almost 2Mhz with dodgy calibration.
When the zero marker is not at the LHS of the screen, the display wraps around in the negative frequency direction.

4) When I eventually get that far how do I use the analyser to see the response of the xtal filter I have just made?

You dont!
At least not without a tracking generator which outputs a signal at the same frequency that the analyser is measuring.

JimB

 

[SwingeyP]

Hello Jim.

Thanks for the reply.

I have created this video to give you an idea of what i'm seeing.

The RF in is 7.0Mhz and the LO is running 3.0 - 3.2Mhz.

I would have expected to see maybe 3Mhz 4Mhz (difference) 7 Mhz and 10 Mhz (sum).

What I actually see is 3, 7 , 10 and what appears to be harmonics of 7.

Could this be that the mixer isn't working?

Regards - Paul

 

[JimB]

OK, here are my rambling thoughts, in no particular order.

The spectrum analyser.
We do not know the amplitude scale calibration. A reasonably inspired guess would say 10dB/division.
The bottom of the trace shows noise, so the 3Mhz is at best 10dB above the noise and the 7Mhz is 15dB above noise. What absolute levels are represented by those numbers is difficult (impossible) to say at this distance. Maybe worth checking using the built in -30dBm 50Mhz cal signal.
Any mixer products will be lower in amplitude than the two inputs and so will likely be hidden in the noise.

What is the input impedance of the analyser?
I am guessing 50Ohm as the legend on the front panel says +10dBm Max (ie 10mW maximum input power).
Could the 50 Ohm input be loading the loading the output of the mixer?
Also, is there DC isolation on the input? Try measuring the input with an ohmmeter, ideally it should show open circuit, otherwise the input of the analyser will upset the DC conditions of the mixer circuit.

The crystal filter.
I assume that this is supposed to be at 4Mhz?
At a quick guess, the filter is one of those build as a crystal ladder with capacitors to ground.
If the filter was working correctly, I would expect the 3Mhz and 7Mhz to completely dissappear at the output of the filter, they are just attenuated a bit.

General comments.
I see lots of LONG wires, totally unscreened, not a hint of a coax cable anywhere. That gives lots of scope for stray coupling between wires, and earth lines which are not low impedance connections to earth due to the inductance of the wires.
I also RF circuits built on plug in breadboards and long wires all around the breadboard.
Breadboards are often (usually) a disaster waiting to happen at RF. (Been there done that and wrote the article https://www.electro-tech-online.com/articles/breadboards-how-bad-are-they-at-rf.278/).

JimB

 

[SwingeyP]

Hi Jim, Thanks for the reply.

Everything you have commented on is spot on. The birds nest and non screened cable in particular. I really started off just testing individual circuits but it seems to have grown. I have found that the supply voltage t the aAD831 is pretty critical. I did manage to get a trace where I saw all of the expected frequencies from the mixer output. Running this from a 9V battery (although just a test) seems to cause a lot of the troubles.

I plan to strip it down to it's individual bits again and prove as I go along. (which is what I started to do in the first place).

Thanks for the help

Paul

 

[JimB]

Try building your prototype on the copper side of a sheet of PCB material.
The copper sheet makes a great low impedance groundplane for earthing and decoupling.

Keep us informed as to how you get on.

JimB