Dishal Filter Alignment with the NanoVNA

When aligning bandpass filters it only takes me a few minutes of twiddling before frustration sets in. I had previously used the method attributed to Dishal as set out in Solid State Design. It is a very effective method although there is some set up needed. 

Over the years I had seen some very vague, if not useless, explanations of how to do this with a VNA. So I set out to discover for myself how this was done.  I threw together a 2 section narrow band pass made with coaxial stubs, tuned with high quality beehive capacitors. In the process I learned not only how to tune these filters but also how to design them with coaxial stubs.

 

Here then is how I use the NanoVNA to align multistage filters. (apologies to keen eyed readers who might noticed the dates/times of these images are mixed up. I lost a few so I had to mix two filter tuning records I had)

1. I used OneofEleven's PC software to drive the nanoVNA but this guide should be near universal.
2. OSL calibration as required
3. Connect the filter to be aligned to the VNA. I'm going to use Channel 0 as the Input, Channel 1 as the output in this guide. 
4. Use a suitable frequency span and add a marker for the desired center frequency eg 146.5MHz
   
5.  Select a polar view of S11 and place a short to ground at Point A
   
   
   
   
   
   
6. Adjust the marker using the "e-delay (ns)" field so it lies on the X axis. This is for convenience, the Y axis would also work. 

   
   

   

   
   

   
   

   
   

   7. Move the short to Point B

   

   
    

   8. Adjust the tuning capacitor on the first resonator until the marker rotates to be on the X axis but opposite to the rotated starting point.

   

   
   9. Move the Short to point C and tune the second resonator
   

   

   
   Remove the short. That's it. The picture in point 9 makes an important point. The whole trace is now on one side of the plot. That's irrelevant. What is important is the marker is on the right hand side of the two points which intersect the X axis. It started on the left hand side in point 8. Just try this, it is easy once you have had a go. Your traces will look very different to mine but the procedure is the same.

   
   So how did it work? As expected:

   

   
   I then decided to rebuild this for a narrower bandwidth. My tuning caps were just a tad too small so I used a center frequency of 150Mhz. You will notice below the peak is at 149.5Mhz. That's because I used a piece of floating PCB onto which the coupling capacitors were soldered. It was getting mechanically depressed during tuning changing the stray capacitance around the filter. 
   

   

   
    
   

   73's
   

    
   

QN8027 FM Stereo Transmitter

The QN8027 is a high performance, low power, full-
featured single-chip stereo FM transmitter designed for
portable audio/video players, automotive accessories, cell
phones, and GPS personal navigation devices. The
QN8027 covers frequencies from 76 MHz to 108 MHz in
50/100/200 kHz step sizes for worldwide FM band support.
The QN8027 also supports RDS/RBDS data transmit.

 

OK, the datasheet makes this device sound interesting. But the datasheet is not really clear on how the device should be used and there is not much information on the web. Regardless, I purchased a few for a minuscule sum and set about interrogating the device with STM8eForth.

I'll publish my code on github soon but for now here is how I investigated the device:

1. bit bang an I2C interface
2. keep a local copy of all the devices registers so I could compare what I send the device with what it accepts and stores
3. some words to send one or more bytes while updating the local register copy
4. some words to retrieve one or all of the devices registers

This allowed me to 

1. reset the device
2. confirm the device registers when reset matched the datasheet
3. change frequency as desired 
4. put the device into transmit
5. send RDS groups
   

It didn't take long before I was successfully shuffling bytes to and fro. In the space of half a day I determined that I had a faulty crystal on one board, since fixed, a functioning FM transmitter (now two with the crystal changed) that I could change the frequency of and that after sending an RDS group the registers did not need to be reloaded if I wanted to resend the group since the contents were still there. 

I need to clean up my code to remove stuff I used for debugging and dispense with my local copy of the device registers.  I plan to post it onto github in a around 5-7 days time.

73's

 

 

 

IF Crystal Filters - NDK Type numbering decoded?

I wanted to know more about the crystal fitlers used in the Tait Orca handhelds ie 45E12AA and 45E15AD. 

I could not find the specifcations for these exact filters. However, after looking at some NDK datasheets I believe these crsytal filter are as follows:

 45 - denotes the nominal center frequency

E - couldn't work the meaning out. Perhazps E means 2 poles and EE means 4 poles?

12/15 - bandwidth inkHz

A  - finalises model number

A/D at the end denotes the center frequency.

 

So, 45E12AA apears to be a 45.1MHz 2 pole crystal filter (seaching for 45E12A did not return a model datasheet, perhaps a special Tait specification?)

and 45E15AD is a 45.1MHz 2 pole crystal filter (search for 45E15A to see the model datasheet)

 

Seems strange that the center frequency is not denoted by a consistent letter.  But as expected when looking for relatively old datasheets, not everything is on the web. 

Best to keep the crystal used to mix the IF down to 455kHz with the filters so you can derive the center frequency if no data can be found.

 

73's

Richard