Tuning RF circuits without a Network Analyzer?

[Mikebits]

Back in my RF days at places like Cubic Communications and Qualcomm, I always had access to proper equipment like Network analyzers to test and tune RF circuits. Now I find myself in a position where I no longer am able to rely on these instruments, and I was wondering what other RF hobbyist do to overcome this expensive obstacle?
One thought I had was to use a bi-directional coupler to measure forward and reverse power, but again I find this method will still require a vector RF voltmeter, not cheap either. I could build RF detectors to give voltage but this would only give magnitude. So I ask ya, what methods do you have in your RF bag of tricks?

Thanks

 

[Mikebits]

I found this configuration. I would skip the ADC and use a meter.

 

[JimB]

I think that the expression "tuning RF circuits" is a bit nebulous and wide ranging.

What type of RF circuits do you want to adjust, and which parameter do you want to optimise?

Finding the resonant frequency of a tuned circuit?
The usual thing here is a "dip oscillator", or if I can really show my age a GDO, a Grid Dip Oscillator.
Although the last time I had to do this, I used the spectrum analyser and tracking generator...
...battered the problem to death with technology, but it made life very easy.

Measuring an unknown frequency?
Today I would just feed it into the counter, or if I wanted a quick and dirty estimate, use the spectrum analyser.
In days gone by, I would listen for it on a general coverage receiver.

Measuring RF power?
In days gone by, measure the voltage across the load using a simple diode detector or a "Valve Voltmeter" and do some calculations.
Today, use an RF power meter based on an AD8307 (I think that is the number), spectrum analyser.
If the power is more than 10mW or so, use various 20dB or 30dB couplers and attenuators.

Tuning a receiver for best sensitivity?
In days gone by, a crystal calibrator using a 100khz crystal. Often getting lost counting the various harmonics to find which frequency you are on.
Today, a real signal generator. Low leakage, accurate attenuator, built in frequency counter, AM and FM modulation.

JimB

 

[Mikebits]

Yes sorry, I should have been more clear. I wish to match an antenna as shown in image. The app note gives starting values but recommends that some tweaking of values is needed based on layout differences and board material etc. It is a simple circuit but it is 868 MHz, so having a good match is important. Not having a network analyzer which is the right way to do this, I am looking for a way to match within my means. Measuring return loss is all I could think of.

 

[JimB]

In that case, have a look at this:

http://miniradiosolutions.com/54-2/

JimB

 

[Mikebits]

Wow, that looks really neat, unfortunately I don't have a use for such a device beyond this one time need so I really can't justify spending $650.00, but if I develop more of a ongoing need, this would be ideal. Thanks

 

[Mikebits]

Jim, Do you have one of those MiniVNA's? If so how do you like it?

 

[JimB]

I do not have a miniVNA, but I do have an AIM4170, which is a single port "Antenna Analyser".

Have a look here:
http://www.ad5x.com/images/Presentations/AIM4170 Review.pdf

I rate it as one of the best bits of equipment that I have ever bought. Opens up a whole new window into the world of RF.

JimB

 

[Mikebits]

That is a real nice looking tool. I just checked on network analyzer rental for 2 weeks. It would be cheaper to buy the miniVNA...
I can rent a mercedes for less than a network analyzer .

 

[JimB]

I can rent a mercedes for less than a network analyzer .

Never mind the Mercedes, when I was on holiday earlier this year I rented this:

Much more fun that a Network Analyser.
I would have tried to bring it back home with me, but the excess baggage charge was a killer.

JimB

 

[Mikebits]

I am surprised they could pry your fingers off the steering wheel

 

[MrAl]

Hello guys,

In all my years i have done very little in radio and always thought about getting into it a little bit more one day. Just an FM transmitter so i could listen to my favorite music on the way to and from work and a simple AM receiver with home built coil is all i ever did in this field really. Oh and a home built, roof mounted, guy wired copper tubing dipole antenna for CB radio back when it was popular, got my signal a few towns over.

A possible simple solution to this problem is to set up a receiver at some distance away. Tune the transmitter for best performance. When i did my FM transmitter i used to take the car out for a drive and see how far i was getting with a particular setup and adjustment. Better adjustment led to greater distance/clearer sound.

 

[Mikebits]

So I am gonna give the reflectometer plan a try, I also found a IC that provides phase and Magnitude. Only problem now is generating 868 MHz. I found a signal solution for $150.00

Key Features:

· Frequency Range is 35MHz to 4.4GHz

· 50 ohm system

· Frequency resolution is 1khz

· RF Pulse Capability (EMC Immunity Testing)

· PLL synthesized RF signal generator, controlled and powered by a PC running Windows via its USB port.

· RF Step Function included

Selectable time base per step and selectable step size.

Steps start and stop frequency can loop or Single Step function

Includes a "Single" step button. Single step forward or backwards

· Improved performance and phase noise (Phase Noise measured at 10KHz offset at 2.4GHz was -88dBc/Hz)

· 4 layer PCB versus 2 layers of similar models. This is a quality made product.

· RF Output Standard SMA Connector: Select 4 Power levels. Maximum outputs are:

1.35MHz - 4GHz = +15dBm +/- 2dB

So let me know what you think? The more I think about it, the better the MiniVNA sounds

 

[JimB]

Mike
Is there a link to the original for the info about that generator?
JimB

 

[Mikebits]

**broken link removed**

 

[JimB]

Wow, that is a lot of signal source for $150.

JimB

 

[Mikebits]

I went ahead and purchased one as he only had 3 left. He has some other ones for $180.0 in a metal box.

 

[Mosaic]

How's that working for u Mike...?
I kinda planned to do this for RF sigs;

Plus I have a Tek SG504 inbound for 'flat' HF's over 245Mhz.

As i invest in outfitting an RF workbench, just stocking on assorted N,SMA and RG58/BNC stuff for proper hookups to attenuators or a Boonton RF power meter can cost a fair amount.

 

[Mikebits]

I kinda got temporarily sidetracked on the reflectometer. The signal source I got works but has a high amount of 2nd and 3rd order distortion which won't work well in my application so I will have to add some filtering. I know what you mean about the price of adapters. I recently purchased some SMA to BNC and other assorted adapters which were quite pricey. The price of cables is pretty high so I will make my own as I use to do pretty well making RF cables. I may purchase a couple high quality SMA cables just for input and output connections.

 

[Mosaic]

I had a chat with your ebay supplier and he sold me one for $150...great guy! Looking forward to using it....seems a great value!
With the Boonton RF 4210 -4b micro wattmeter and a Tek SG504 flat response sig gen (245 Mhz to 1+ Ghz) plus my DSA 815 Tg , I am beginning get some RF capability and the ability to cross check results/measurements using different methods.
I have a couple JFW step attenuators (SMA) coming in as well...10dB and 70dB good to 2.2Ghz.
I should be able to make custom attenuators as well which I can calibrate (Pi or T) , also these attenuator chips look interesting..HMC654LP2E

Ebay has good value on stuff like SMA end launchers etc.
https://www.ebay.com/itm/221549602889?_trksid=p2057872.m2749.l2649&ssPageName=STRK:MEBIDX:IT