Gamma Dog - Rate-to-Tone conversion and Audio Frequency Modifier

One of Gamma Dog's unique features is the continuously variable tone representing the detected count rate by changing its frequency.

The approach is fairly straight forward - the detected count rate in CPS (Counts-per-second) coming out of the detector and into the MCU is converted into an audio tone with the same frequency - i.e. 200 CPS will produce 200Hz tone and 1000 CPS will produce 1 kHz tone. 

Due to a lucky coincidence, the detectors we use, especially the 63mm NaI(Tl) crystals produce around 180 - 230 CPS as a background level which is a really good starting point and the  overall detector sensitivity and rate response work pretty well with such direct conversion.

As the count rate increases though, the tone frequency will increase (as expected) and this could become a problem at some point when the count rate gets really high (above 7000 - 8000 CPS) - nobody likes these very high pitch audio frequencies (certainly not the dogs and the mine bats :-) - such high pitch is not the most pleasant thing to listen to. Not to mention it becomes quite difficult to hear small variations in the frequency within this high frequency range.

To combat this problem, the "classic" version of the Gamma Dog always starts the frequency generation at the set squelch level - this way if Squelch is set to 7000 CPS, and detected rate is around 7000 CPS the tone frequency will be low - less than a hundred Hertz (whatever difference is needed to break through the Squelch Threshold) , opposed to a 7 kHz tone.

In my Gamma Dog+, the system for rate-to-tone conversion is further improved and offers the user a toolset of numerous conversion options. These options afford greater control over how the Frequency Audio Response to the Count Rate conversion is taking place, customizing it for different applications and listening preferences. 

The "xL" indicates Logarithmic Scale conversion (selectable when in Continuously Open Squelch Mode / "#" by using the Soft Key) 

Changes in the conversion model are done via a user-selectable option that can be assigned to the Soft-Key button. It is called Audio Frequency Modifier or AFM for short. 

In a nutshell, AFM (in some of the options) is a Multiplication Factor that is applied to the count rate while converted to tone:

There is a total of 8 options: x0.5, x1 (default), x1.25, x1.5, x2, Auto, Exp and Log

The first few options are self-explanatory - if x0.5 is selected, the rate is divided by 2 before it determines the frequency of the tone - i.e. 200 CPS rate will produce 100Hz tone, 210 CPS will produce 105 Hz and so forth. It halves the base frequency but also the steps between changes. On the other end, with x2 selected, 200 CPS will produce a 400 Hz tone and 210 CPS will produce a 420 Hz tone. The default value of x1 is the direct 1:1 conversion.

The x0.5 option for example, is useful with very large detectors to keep the audio frequency output low against the natural high-count rate of the detector, while x2 is useful with smaller detectors which natively produce fewer counts, and the option allows to keep the tone frequency higher than a direct 1:1 conversion in this case.

In Auto mode the multiplication factor is based on the Detected Rate increase over the Squelch Threshold Level. The audio frequency modifier is dynamically adjusted in 4 steps based on the delta between the two rates.

If Current Rate exceeds Squelch Rate by more than 175% - Frequency Multiplier x2.5 is used.

If Current Rate exceeds Squelch Rate by more than 150% - Frequency Multiplier x2 is used.

If Current Rate exceeds Squelch Rate by more than 125% - Frequency Multiplier x1.5 is used.

If Current Rate exceeds Squelch Rate by less than 125% - Frequency Multiplier x1 .25 is used.

This feature will change automatically through different multiplier levels while using the squelch level as control of where the "step-ups" should take place.

This graph plots how the frequency conversion steps through the multiplication range using the difference between detected rate and the set squelch rate. As the squelch opens and the rate continues to climb, the multiplier will start stepping up, increasing the audio frequency.

There are also two non-linear conversion modes available - Exponential and Logarithmic.

Exponential Mode – The audio tone frequency will increase in an exponential manner, with a scale factor of 0.0033 and base frequency of 100Hz using (e) Euler's Number.

This feature is independent of the Squelch Level - the squelch just needs to open but otherwise has no effect on the conversion. 

It is usable with absolute rates up to 1400 CPS.  Beyond 1400 CPS the audio frequency will exceed 10kHz!

Exponential mode is useful to detect very small increases in the count rate when the absolute rate is also very low. For example, in very low activity areas where small rate changes need to be detected - the audio tone frequency is exponentially increased, exaggerating the tiny rate variations by using higher pitch tones.

Logarithmic Mode – The audio tone frequency will change on a Logarithmic scale – the range of 40 CPS to 10K CPS will be "compressed" and mapped by using a logarithmic curve to 40Hz – 3kHz audio range.

This feature, just like Exp Mode is independent of the Squelch Level from Rate-To-Frequency standpoint, and it provides very good audio resolution for lower count rates (<2000 CPS), while still capable of handling very high count-rates (2K to10K+ CPS) - all within a manageable 3kHz audio range.

 

The range between 2000 CPS and 10000 CPS is allocated within less than 1kHz audio frequency response (from ~2200 kHz to 3000 kHz) which is useful when the instrument is used with both, very low and very high-count rates. This comes at the expense of audio resolution in the high-count region of the curve and overall higher frequency tone at the mid-low count range.

In the Gamma Dog's menu system, there is a config item (#10) which allows the user to select a startup mode for the Audio Frequency Modifier, but the mode can always be changed later, during operation, if "Multiplier" is assigned to the Soft Key button in Menu Item #8.

(The other assignable function to the Soft Key button is control for the Audio Conversion Hysteresis Level responsible for how closely variations of the rate are followed before converted to audio tone)  

Efficient Common-Mode Current Choke for EFRW and other portable antennas (1:1 Guanella)

I was putting together SOTA/POTA portable antenna kit for my son, utilizing EFHW and EFRW antennas and I needed a Common-Mode Current Choke - something not very large and efficient that he can eventually use from QRP to 100W on 40m to 6m.

This is just a quick, easy, 15 min. build, and the result is an excellent and pretty efficient CMC Choke (see the measurements below) for only $25, that one can place in-line with the antenna feedline at the transceiver, at the antenna feed-point or both.

There is nothing new here - this is "Classic" 1:1 Guanella choke build, but I did some measurements to put things in perspective and show what is to be expected from such choke.

Note regarding using a CMC choke with portable wire antennas - if used with End-Fed Random Wire (EFRW) the choke can be placed at the antenna feed point (the 9:1 UnUn transformer) or at the radio. When used with End-Fed Half-Wave (EFHW) the choke should be placed at the radio only as the coax is part of the antenna system.

The bill of materials includes FT-240-43 toroid core (source: Amazon, $12) and 5 feet of RG-316 coaxial cable with pre-installed Male and Female BNC connectors (source: eBay, $13). Other materials - 6 small zip-ties and a piece of wide heat-shrink tubing which I had laying around.

The coax is winded on the core in 2x 11 turns with a crossover turn as shown. The total is 23 turns through the core.

 First, I secured one end of the coax with 2 crossed zip-ties, wound the first 11 turns, added zip-ties for the cross-over turn to fix each side of the windings and finished it with another 11 turns and 2 crossed zip-ties.

The core of this size can easily take more turns - around 14 on each side (for total of 29) but there is a performance trade-off - more turns improve low frequency attenuation but due to capacitive coupling between the tightly spaced turns, this will decrease the attenuation factor for higher frequencies. 

It seems that for FT-240 core and RG-316 the "sweet spot" is around 11-12 turns on each side. If 12 turns are to be used, one needs 6 feet piece of coax. 

The turns must be fairly tight around the core itself, so they don't slide around (PTE-insulated coax is very slippery!) but not too tight to cause coax damage. Turns should be separated from each-other as much as possible.

The direction of the windings doesn't matter. The purpose of the Reisert Crossover turn is to allow the coax to leave the core from opposing sides so there is no coupling between input and output and makes it mechanically better for in-line placement but electrically both sides are the same as one continuous winding.

The cores I got from Amazon had rounded edges so there was no need to wrap the cores with tape but should the core has sharp edges, fiberglass tape must be used before winding.

The complete CMC Choke.  With 5 feet of coax there are approximately 4" left as pigtails on each side.

Measurements

I built the CMC Choke Test fixture on a piece of FR4 material. Two SMA connectors are soldered on each side of a ground plane and a short Teflon-insulated wire with alligator-clips is soldered to the center conductor of each connector.

A piece of high-density foam glued at the front edge of the board prevents the wires from getting too close to the ground plane during calibration and measurements.

I added a 50 Ohm LOAD (2x 100Ohm 1206 SMD resistors in parallel) to be used at part of the VNA's OSLT calibration procedure.

OPEN calibration is done with the clips just laying far apart from each other. SHORT is done by clipping the S11 (Ch. 0) of the nanoVNA to the ground plane.

LOAD calibration on S11 is using the built-in LOAD standard.

For the THRU calibration, used for the S21 measurements both clips are attached together.

To measure the attenuation provided by the CMC Choke, the alligator clips are attached only to the outer shielding of the coax.

The CMC choke's performance is excellent for the entire intended range - 160m to 6m!

For reference - anything below -20dB is very good and below -30db is considered excellent!

Marker 1 (80m band @3.5 MHz) -44.7dB.

Marker 2 (20m band @14.2MHz) -38.9dB

Marker 3 (10m band @28.5MHz) -30.8dB

Marker 4 (6m band @54.1 MHz) -19.5dB

Performance on the 6m band while still very good is a bit lower than the lower HF range due to the coupling between multiple turns. As I mentioned, this an expected trade-off with the lower bands.

As the VNA shows, on 20m the impedance (|Z|) is over 8KOhms and even higher on lower frequencies. Not too shabby!

In an attempt to test and improve further the performance on 6m, I added a split-core mix 31 bead with 3 turns to one of the pigtails.

The 3-turn split-core bead definitely improved the 6m band attenuation by -7dB, down to -26.5dB.
This is a "good-to-know option" - I probably will keep the bead in my kit as an "add-on" for 6m. The improvement from this additional core on the lower frequencies is just a few dB over an already excellent performance so there is no need to be a permanent addition.

S21 measurement for insertion loss. Since the two SMA-to-BNC adapters are M and F on the BNC side, just coupling them together for the THRU S21 calibration then yields a pretty accurate measurement.

The insertion loss is mainly due to the 5 ft. of small diameter coax (RG-316) and it is absolutely acceptable with less than -0.2dB on 20m and -0.3dB on 6m.

The finished chokes looking like hockey pucks.

A piece of heatshrink tubing fixes the coax turns in place and adds a layer of protection.