Tuesday, June 16, 2015

SNA Jr Software update two

I changed the display from a 1.44" 128x128  TFT ti a 1.8" 160.x128 TFT.  Unfortunately they use different driver chips so I had to make some changes to the software , mainly a different driver library and the position of text on the screen.  While I was doing that I also added a couple of
functions to the system.

New circuit board layout moved parts around to make room for the larger display, and added a ERA-3 MMIC amplifier  with attenuator to boost output and present 50 ohm to the DUT. 




After I got the hardware working with the pickup coil to use the SNA Jr. as a Dip Meter, I decided to add a DIP function to the software.  It does a sweep from 1 Mhz. to 50 Mhz. with 50 KHz steps, and looks for the deepest dip in the signal from the RLB.  After that sweep it sets the  range  to 1 MHz.  below and above the dip frequency and then  does a normalized sweep of that frequency range.  Much faster than sweeping through in several ranges.


In the existing sweep functions, they are a single sweep triggered by a short push on the encoder button.  After the sweep is started pressing and holding the button down will enter a setup mode, where you can make adjustments to the gain pots for the sweep range you are using if desired.  Because I added the normalizing  function, I had to do a complete  sweep and then display the data.

video
Moving the coil wound on the pen with parallel cap
in and out of the "Dip Meter" pickup coil


From comments on  QRP-L,  sometimes a continuous reoccurring sweep would be better. This would be useful for adjusting filters, and other alignment procedures.

I added a FAST sweep function that does sweeps at around 1 second per sweep. To get this rate I had to remove the setup functionality, and only did one reading at each point instead of using multiple readings to reduce noise. In this FAST mode I erase the old data point and draw the new datapoint as it is acquired.

Still waiting on some parts to arrive, before I can finish testing the updated circuit board layout.  After the last local QRP club meeting I was asked about the possibility of making kits available.  Still trying to decide if I want to go this route, or just make the design for boards and software available.  If anyone would be interested in a kit of this project drop me a e-mail at duwayne@kv4qb.us   Depending on the response I will decide which way I will go.


Wednesday, June 3, 2015

SNA Jr as Grid Dip Meter

One of my projects needs a 70Mhz. Low Pass Filter.  I plan on using air wound coils for its construction.  Only problem is that my RLC meter does not measure down to the inductance values I need.  I remember reading somewhere that they used a RLB with a pickup coil to get similar results to what you would have with a Grid Dip Meter.



I made up a small pick up coil using an old plastic 35mm film canister, 4 turns of wire and a SMA connector. I connecting it to the RLB and SNA jr.  I quickly wound about a few turns of wire around a old pen, and soldered a 330pf cap across the coil.  The film canister is large enough that I could insert the coil and capacitor inside for maximum coupling.


 Doing a couple of wide band sweeps in the normalized mode I saw a small dip in the signal around 10 MHz.  Narrowing the sweep I found a nice dip and measured  9.259MHz.  Since I knew the value of the parallel capacitor, I plugged those values into an online LC resonance calculator.  Came up with a value of 1.01uh for the coil.  Squeezing and spreading out the coil I came up with values that looked appropriate.  With this method I should be able to wind the coils I need for the LPF.  And have another tool to play with.



UPDATE:

The   pick-up coil worked well for air wound coils, but I wondered if I could modify it for doing toroid coils?  Looking at the manual for a Grid Dip Oscillator, I saw that for toroid coils they used a coupling loop around the GDO coil  connected through a couple of turns through the toroid.   I wound a few turns of wire around the end of the film canister next to the pickup coil ,and soldered the ends to a couple of pins from a good quality IC socket.  I knew these pins would be a good fit for the connectors on the solderless bread-board jumpers I have.  The connectors on these jumpers are also small enough to fit through the toroid with windings in place.


I  wound up a toroid coil and soldered a 47pf capacitor across it.   Running the bread-board jumper through the toroid a couple times I connected to the pins on the modified pickup coil.  Did a wide band sweep and saw a little dip around 20 Mhz.  Narrowing the sweep I was able to find a dip at19.819 Mhz  This dip is not as deep as with the air wound coil, but is deep enough to use.


Using the LC Resonant frequency calculator, I got a value of  1.37 uH.  Going to a toroid coil calculator for the type of core I was using I entered the value of capacitor and number of turns in the coil.  This gave me design values of 20.18 Mhz. and 1.26 uH.  Looks like this simple pickup will work for both air-wound and toroidal coils.