Another method of measuring video carriers with an HF/VHF synthesised receiver

Todd Emslie



Introduction

TV DXers are often able to identify weak video carriers by using precision frequency measurement (PFM). This article describes one of several methods which can be used for measuring TV video carrier frequencies. Having said that, there are limitations in this measuring method. For example, it is only suitable for VHF AM/FM/SSB receivers covering both HF and VHF. Also, if your receiver suffers from poor frequency stability, the following method will give false measurements.

The following method of measuring carrier TV frequencies is suited to TV DXers who do not have access to a reliable accurate frequency reference. The method assumes that the DXer has to rely on the 5, 10, and 15 MHz WWV time and reference HF signals.

Measurement method, using a digital audio frequency meter

Note: Before proceeding with the following method, you need to measure the audio frequency on your Icom R8500 for WWV on 4.999 MHz, 10.999 MHz, and 14.999 MHz USB. You can measure theses carriers by using a FFT type spectrum analyser (Spectrum Lab, etc), or a standard DFM (digital frequency meter).

Ideally, WWV 14.999 MHz USB would correspond to an exact 1,000 Hz tone. However, in the real world, different radios will provide different beat tones when tuned to 14.999 MHz USB. For example, while my IC-R8500 provides a 1012 Hz beat tone at 14.999 MHz USB, another radio I own provides a 1060 Hz tone at 14.999 MHz USB.

When tuned to WWV, if your receiver shows 800 Hz with the receiver dial on 14.999 MHz, it means that the receiver is 200 Hz high, while an audio frequency of 1200 Hz means that the receiver is 200 Hz low. Then apportion the error to the frequency being measured, i.e., in the case of chE2 TV, 48.25/15 MHz x frequency error.

By measuring WWV 4.999, 10.999, and 14.999 MHz USB, you can determine the incremental slope frequency error from 4.999-14.999 MHz. You can then extrapolate the slope error for band I TV carriers above 45 MHz.

Let's assume a particular receiver has an incremental slope error of +4.5 Hz per MHz, a table could be compiled similar to the one below:

RF carrier USB Frequency offset Frequency:

14.9990 MHz WWV 1055 Hz.
45.2490 MHz NZch1 1194 Hz.
46.2390 MHz Auch0 1196 Hz.
48.2490 MHz chE2 1205 Hz.
49.7490 MHz chC1 1211 Hz.
55.2490 MHz chA2 / E3 1236 Hz.
57.2499 MHz chAu1 1245 Hz.
57.7490 MHz chC2 1247 Hz.
59.2490 MHz chR2 1254 Hz.
61.2490 MHz chA3 1263 Hz.
62.2490 MHz chE4 1268 Hz.
64.2490 MHz chAu2 1277 Hz.
65.7490 MHz chC3 1283 Hz.
67.2490 MHz chA4 1290 Hz.

Because your receiver will have its own audio beat tone for WWV 14.999 MHz USB, you have to make up your own table. You then have to calculate the frequency slope error by comparing the data points for WWV 4.999, 9.999, and 14.999 MHz.

Using WWV 15 MHz as a reference, actual example

1. 14.999 MHz WWV USB reading = 1015 Hz.

2. In USB, tune the DX TV RF carrier until you obtain a similar reading to 1015 Hz.

3. Take note of the RF TV carrier: 46.23908 MHz = 1019 Hz USB.

4. If the tuning steps in the receiver did not enable an exact 1015 Hz reading, subtract or add the difference. For example, the RF TV carrier was too low: 46.23908 MHz (1019 Hz). In this case, we need to add 4 Hz to the carrier: 46.23908 + .000004 = 46.239084 MHz.

If the RF TV carrier is too high, for example, 46.23908 (1010 Hz), we need to subtract 4 Hz from the TV carrier: 46.23908 - .000004 = 46.239076 MHz.

5. Apply the simple formula: WWV error x ( RF TV carrier / WWV RF carrier ) or: 15 Hz x (46.239084 / 15) = 46.239084 Hz (rounded figure = 46 Hz).

6. Subtract 46 Hz - 15 Hz = 31 Hz. This is the correction factor in Hz for the TV RF carrier.

7. Add 1 KHz to the RF TV carrier: 46.239084 MHz + .001 MHz = 46.240084 MHz.

8. Deduct the 31 Hz correction factor: 46.240084 MHz - .000031 = 46.240053 MHz.

9. The TV DX RF carrier = 46.240053 MHz.

1. Measure the USB audio frequency of 14.9990 MHz WWV.
2. Tune the DX TV RF video carrier, approximately 1 KHz below the '0' beat frequency in USB, and measure the audio frequency.
3. Take note of the RF frequency displayed on the R-8500, and add the apparent audio frequency.
4. Then look at the offset reference list, and subtract the actual offset in Hz.

Example: ABMN0 video is received via TEP.
1. Select a memory pre-set, or priority channel for WWV on 14.9990 MHz USB, and measure the audio frequency.
2. Then tune the ch0 video carrier using USB and 10 Hz steps, to approximately 1 KHz (46.2390 MHz) below the video carrier, and measure the audio frequency (1080 Hz).
3. Take note of the RF frequency (46.2390 MHz) displayed on the R-8500, and then add the audio frequency (1080 Hz). 46.2390 MHz + 1080 Hz = 46.240080 MHz.
4. Then look at your offset reference list, and subtract 26 Hz (46.24008 - 26 = 46.240054 MHz.
5. You should arrive at a figure between 46.240050 MHz and 46.240060 MHz. This represents accuracy within +/- 5 Hz.

Method 2 (no digital frequency meter required)

1. Select a memory preset or priority channel for WWV on 14.9990 MHz USB.
2. Then very carefully tune the DXTV video carrier, using USB and 10 Hz steps, until you obtain a similar audio pitch to 14.9990 WWV.
3. Take note of the RF frequency displayed on the R-8500, and add 1 KHz.
4. Then look at the offset reference list, and subtract the actual Hz offset.

Practical example 1

ABMN0 channel 0 Wagga is received.
1. Select a memory preset or priority channel for WWV on 14.999 MHz USB.
2. By carefully tuning the ABMN0 carrier until I obtained a similar audio pitch to 14.999 MHz USB WWV.
3. I took note of the RF frequency of ABMN0 (46.239080 MHz). I then added 1 KHz from this measurement (46.239080 MHz + 1000 KHz = 46.240080 MHz).
4. I then subtracted the 25 Hz offset (see table below).
5. 46.240080 MHz - .000025 = 46.240055 MHz.
6. We have measured the 46.240055 MHz video carrier of ABMN0.

Note: These audio measurements are all based on a 12 Hz reading for 14.999 MHz USB. The ratio table is used when the audio frequency for WWV on 14.999 USB varies. For example, if WWV 14.999 reads 13 Hz, and we are measuring a DXTV carrier on approximately 45.25 MHz, multiply 3.058 X 13 = 39.75 Hz.

It is advisable to install the Icom CR-293 high stability crystal unit. This will minimize the frequency drift at 15 MHz to ~ +/- 2Hz (+/- 6Hz @ 48 MHz). If your IC R-8500 is not fitted with this crystal, you will need to leave the radio on for at least ~ 15 minutes before taking frequency measurements.

Practical example 2

chE2 Nakhon Ratchasima, Thailand is received via TEP.
1. Select a memory pre-set or priority channel for WWV on 14.9990 MHz USB.
2. Then carefully tune the chE2 signal using USB and 10 Hz steps until you obtain a similar audio pitch to WWV.
3. Take note of the RF frequency displayed on the R-8500, and then add 1 KHz.
4. Then look at your offset reference list, and subtract 26 Hz.
5. You should arrive at a figure between 48.23958 and 48.23960 MHz.

The above method will give a slightly more accurate measurement if a digital frequency meter is used.

I have found it convenient to place 14.9990 MHz USB in the IC-R8500's priority memory channel. I then use this as a instant comparison.

Since installing the Icom CR-293 high stability crystal (+/- 0.5 ppm at 0-60 C), the audio reading on 14.9990 USB, has varied between 11-13 Hz (12 HZ +/- 1 Hz. This translates to 38.3 Hz +/- 3.2 Hz at 48 MHz.

I have measured the frequency offset errors versus actual frequencies on my Icom IC R8500 receiver, using a 15625 Hz TV-derived reference unit, and digital frequency meter (see table below). The initial frequency error at 15 MHz increases by about 0.8 Hz per 1 MHz increase in frequency.

Table of frequency measurements

RF carrier USB Apparent audio Ratio Actual offset .
14.9990 MHz WWV 1012 Hz 0 0 Hz .
45.2490 MHz NZch1 1036.7 Hz 3.058 24.7 Hz .
46.2490 MHz Auch0 1037.5 Hz 3.125 25.5 Hz .
48.2490 MHz chE2 1038.3 Hz 3.2583 26.3 Hz .
49.7490 MHz chC1 1040.3 Hz 3.3583 28.3 Hz .
55.2490 MHz chA2 / E3 1044.8 Hz 3.7333 32.8 Hz .
57.2499 MHz chAu1 1046.4 Hz 3.8666 34.4 Hz .
57.7490 MHz chC2 1046.9 Hz 3.9083 34.9 Hz .
59.2490 MHz chR2 1048.0 Hz 4 36.0 Hz .
61.2490 MHz chA3 1049.6 Hz 4.1333 37.6 Hz .
62.2490 MHz chE4 1050.4 Hz 4.2 38.4 Hz .
64.2490 MHz chAu2 1052.1 Hz 4.3416 40.1 Hz .
65.7490 MHz chC3 1053.4 Hz 4.45 41.4 Hz .
67.2490 MHz chA4 1054.5 Hz 4.5416 42.5 Hz .
77.2490 MHz chA5 / C4 1062.7 Hz 5.225 50.7 Hz .
83.2490 MHz chA6 1067.6 Hz 5.6333 55.6 Hz .
85.2490 MHz chC5 1069.2 Hz 5.7666 57.2 Hz .
86.2490 MHz chAu3 1070.0 Hz 5.8333 58.0 Hz .
87.2690 MHz chAu3 1070.9 Hz 5.9083 58.9 Hz .
91.2490 MHz chJ1 1074.1 Hz 6.175 62.1 Hz .
93.2490 MHz chC6 1075.7 Hz 6.3083 63.7 Hz .
95.2490 MHz chAu4 1077.4 Hz 6.45 65.4 Hz .
97.2490 MHz chJ2 1079.0 Hz 6.5833 67.0 Hz .
102.2490 MHz chAu5 1083.0 Hz 6.9166 71.0 Hz .
103.2490 MHz chJ3 1083.9 Hz 6.9916 71.9 Hz .

Links

Accurate Measurement of AM/SSB and TV Carrier Frequencies. Ian Roberts, ZS6BTE.

Summary article for measuring TV DX video carriers. Todd Emslie.

DIGIPAN software for accurate measurements of TV DX video frequencies.

Precision frequency measurement of DX TV carriers. Kunihiko NAKANO, JE7IDA (Japan).