April 2014 FMT Actual Results and Analysis
This article will discuss the ARRL April 2014 FMT prupose, methodes used, station configuration, results obtained for each method, and then a discussion on the expectatons of both methods, results, and some of the "whys". This radio used is a SDR DDC type radio. Measurement methods can be fundimentaly different than typically analog radios. The findings are interestingly different considering both methods used a DDC digital radio.
Overview
Frequency Measurement Tests, FMT, are done to test the operational and technical skills all amateur radio operators are expected and assumed to have mastered. Amateurs must know what frequency they are "transmitting" to ensure the "modulated" or "unmodulated" carrier is "totally within" the authorized band segment their license authorizes. The FCC has zero tolerance for violations. The bands are defined such that "only" Extra Class License holds may operate near the "band edges". That means Extra's are protectors of the band edges. However, Technicians and Generals have defined operating edges within each frequency band.
All of this is a throwback from the days of tubes and drifting oscillators. Today’s radios are higher quality and more frequency stable, typically 1-3 parts per million, meaning about 15-60 hz at 14Mhz. Your transmitter's carrier could be 15 hz plus or minus what your dial and display are indicating. Any modulation of the carrier also. It would not be difficult to set you transmitter up in SSB USB with a 2khz filter on 14,348,000 Mhz and be operating out of your authorized band by 15hz. You are held accountable for violations if just 0.001 hz out of band!
Many hams have never measured a frequency and never know if they are transmitting out of band. It is easy to "miss" any questions on license exam dealing with this issues and still past the exam.
There are many methods to measure a frequency and then know the "margin of error" your transmitter has, and thus avoid operating too close to the band edge.
The FMT is a good test to explore all the different methods and build your skills in this area. Some are very low tech and other high tech; it doesn't matter, they all produce a close result, giving you an excellent idea of your transmitter’s frequency accuracy. The methods differ enough, they will provide some applied learnings in how our radios work.
Below is are two examples of measurement techniques using the "identical" radio and setup. Two different results that are very close to the actual FMT frequency. The radio used is a DDC SDR, state of the art radio. The same methods can be used with typical analog solid state radios, but with different results.
The purpose of the FMT is to understand your "margin" of error and adjust your operations when close to any band edge. Some hams become intrigued with methods that are accurate to within .001 Hz. At this degree of accuracy, Doppler is the largest variable in accuracy. Doppler and other propagation effects can be very interesting and many hams are dedicated to understanding and measuring these effects.
FMT exercises will give you a better understanding of how your radio works and insite into propagation.
Read through the following methods, they appear simple and maybe even intuitively obvious, but actually performing these methods can be challenging.
The FMT effort for W7MOT and AA7OO was lead by Norm Johnson, AA7OO.
Actual Radio Frequencies (Hz):
80m 40m 20m
W100AW/5 3,598,137.74 7,058,632.37 14,121,135.32
Submitted Results under the call signs AA7OO and W7MOT ; includes the Delta from W100AW/5 Actuals.
AA7OO AZ W7
Results:
Band Submitted Delta from W100AW/5
80m : 3,598,138.82 1.08
40m : 7,058,630.94 -1.43
20m : 14,121,137.09 1.77
Station Configuration:
Radio (SDR): SDRStick HF2 DDC Receiver
Computer: ASUS AC-N56RJ
Software(SDR): HDSDR
Software (audio): SpecLab
Network: Netgear 1GB Switch
1GB ASUS AC-RT66R Router
Antenna: Scorpion mounted at 8ft with elevated ground plane.
10MHz standard: Trimble GPSDO
Variable Signal Generator: Motorola R-2600, 1xE-11 with External GPSDO
Measurement Method:
Set R2600 Gen less than 2khz below expected FMT freq.
Using HF2, recorded 1.7Mhz of RF band centered 2Khz below R2600. Three recordings were made, one for each FMT run, starting 5 minutes prior and 5 minutes post each band FMT schedule.
In "real time", calibrated HF2 RF Front End 5 minutes prior to FMT Start Time.
HF2 Tuned to R2600 & measured Ref freq at sub 1hz resolution.
During FMT Key Down, measured FMT freq and noted for reference.
All radio setting; LO, Tune, Output Bandwidth, filter, etc.
Post analysis is done by playback of FMT band recordings using the HF2 receiver for playback. There are many different set-up and procedural options to choose in combination with a DDC receiver and SpecLab that will result in the same result, measurement of the FMT frequency.
For POST analysis, playback HF2 audio output is streamed via a virtual audio cable to SpecLab for audio analysis.
Three steps are performed during the playback,
set-up of HF2 audio,
set-up of SpecLab,
and generating FFT export file.
First, the HF2 is Tuned such that the audio output contains the R2600 audio_ref signal and audio_FMT signal. RF and Audio filters are applied to reduce nearby interfering RF & audio signals.
Second, SpecLab is set-up to view the same HF2 audio output and the FFT export defined to capture the audio_ref & audio_FMT.
Third, SpecLab FFT export is manually started and stopped for FMT Key Down. The Export FFT data is then processed to determine the audio_Ref and audio_FMT.
Techniques used to qualify statistically relevant data points.
Using the same techniques to eliminate any variations in the receiver processing; a calibration factor is defined as the delta of R2600 Gen Ref and measured Ref. The difference of audio_ref and audio_fmt is then added to the correction factor which is added to the R2600 Gen Ref, resulting in the post FMT measurement results.
Soapbox:
Enjoyed the FMT. Used a newly acquired DDC SDRStick HF2 receiver. An FMT forces one to learn how these DDC receiver really works, its all about the math. Very well executed FMT, spot on with the schedule.
Band conditions were favourable to receive all FMT runs in Arizona. During each run there were some noted interference signals, but nothing that caused a real problem or concern. All FMT signals were received at least 5 db above noise; 20M was 10 db above noise; 40M was 22 db above noise; and 80M was 6 db above noise.
STDev.p of raw data was 0.7 , 0.9 , and 1.1 respectively.
W7MOT AZ W7
Results:
Band Submitted Delta from W100AW/5
80m : 3,598,137.00 -0.74
40m : 7,058,632.00 -0.37
20m : 14,121,136.00 0.68
Station Configuration:
Radio (SDR): SDRStick HF2 DDC Receiver
Computer: ASUS AC-N56RJ
Software(SDR): HDSDR
Software (audio): SpecLab
Network: Netgear 1GB Switch
1GB ASUS AC-RT66R Router
Antenna: Scorpion mounted at 8ft with elevated ground plane.
10MHz standard: Trimble GPSDO
Variable Signal Generator: Motorola R-2600, 1xE-11 with External GPSDO
Measurement Method:
This is a "real time" direct measurement method of the FMT frequency using a DDC SDRStick HF2 receiver.
Setup the R2600 Gen to something less than 2khz below expected FMT freq.
On HF2 HDSDR, set LO about 4 Khz below R2600.
In "real time", calibrate HF2 HDSDR RF Front End about 5 minutes prior to FMT run Start Time.
After calibration, Setting HF2 HDSDR Tune to R2600 frequency, measure Ref freq to 1hz resolution, limitation is HDSDR displays only 1hz resolution. The frequency readout should agree with R2600 frequency.
Real time measurements are done using two methods, HDSDR mode set to ECSS and then CW. Both methods should gave the same result, even though the techniques are different.
During FMT Key Down,
ECSS Mode:
set HF2 mode to ECSS,
set LO close to FMT frequency,
allow Tune to seek and lock on signal.
Directly recorded FMT frequency.
CW Mode:
set HF2 mode to CW.
set LO close to FMT frequency.
directly read FMT frequency.
Compare the ECSS and CW readings, they should be equal.
FMT frequency result is the CW reading, which matches the ECSS reading.
Soapbox:
This submission is under the W7MOT club call using the same station set-up as AA7OO. The method used is the simplest and fasted option, directly reading the FMT frequency from the radio's display. The reading remained constant over the FMT period.
The intent of this alternative measurement is to demonstrate & compare in the heat of competition two different method results. See the AA7OO results to compare. Note, the direct method is less than 2hz difference from AA7OO which is about the same as the correction factor calculated in AA7OO.
Logic would suggests the "direct readout" method would be closer than the "off-set audio analysis" method. The direct measurement is done after receiver calibration and the HDSDR display calculates the frequency using the RF digitized data upstream of the processed "digitized off-set audio" used by SpecLab software. We shall see and evaluate both methods after FMT actuals are posted.
Analysis