Hermes-Lite TX Spectrum for 12M

These measurments were done with a Hermes-Lite v1.22 with 73.728 MHz oscillator, built in the standard way, unfiltered, unamplified and operating at the power indicated. The tone was generated with Quisk's spot function.

Unaltered Spectrum

Below is the unaltered spectrum when transmitting on 24.895 MHz. The 12M band was chosen as it was identified by several as the most problematic. It is not as easy as the 160M example to identify sources for all the spurs, but the same categorization of spurs and artifical removal may be done. Note that the most problematic spur for 12M is seen at 22.98 MHz, which is 2Fs - 5Ftx, or 2(73.782) - 5(24.895).

Altered Spectrum

Below is the spectrum after all three categories of spurious emissions have been removed: natural harmonics, scope artifacts and Ftx alias, nFs plus or minus mFtx spurs. The extracted values for these emissions appears later in tables. Note that most remaining emissions are less than 70 dBc and/or relatively higher than Ftx.

Natural Harmonics

Below are all natural harmonics starting with the 2nd for TX frequency of 24.895. Measurements for 5 different output power levels are made. Note that the odd harmonics are stronger than the even harmonics. Note that the biggest reduction in harmonics appears to occur between 14 and 17 dBm output.

MHz	20.4	20.1	17.4	14.4	11.5
49.79	-45.2	-45.4	-41.9	-50.7	-51.4
74.69	-33.3	-34.5	-38.8	-44.7	-49.9
99.58	-63.0	-61.4	-57.1
124.48	-48.8	-50.7	-62.8

Scope Artifacts and Ftx Alias

Below are artifacts introduced by mixing with an oscillator internal to the scope, plus the expected Ftx alias at the end of the spectrum due to the scope's 1GHz ADC. These do not exist in reality. Note that they remain relatively constant for all power output levels.

MHz	20.4	20.1	17.4	14.4	11.5
62.50	-60.0	-58.3		-64.8	-61.7
125.00	-63.4
225.10	-58.0	-59.8	-57.8	-57.6	-57.8
250.00	-57.6	-58.7	-62.7		-63.3
274.90	-57.9	-57.7	-57.4	-57.4	-57.6
350.10	-58.0	-57.6	-57.4	-57.5	-57.2
375.00	-55.8	-56.0	-60.7		-60.1
399.90	-57.0	-56.6	-56.5	-56.5	-56.3
475.10	-48.8	-48.8	-48.8	-48.7	-48.7

Spurs at nFs plus or minus mFtx

These are the most problematic emissions seen, but are not unexpected given the DAC, DAC operation frequency and interpolation filter.

First, note the strongest spur at 2Fs - Ftx, or 122.56 MHz. Given the AD9866's 2x interpolation filter and sampling rate of 73.728 MHz, this is entirely normal. This is why reconstruction filters are required when using a DAC. Fortunately, this ~5 times Ftx and should be easy to filter. Also, these spurs will move up by another ~6 MHz when the AD9866 Fs is raised in V2.0. The power level of this spur relative to the fundamental does not change much when varying the TX power as expected. A similar strong spur is seen at 2*Fs + Ftx, or 172.35 MHz.

We also see spurs at 2Fs - mFtx for m of 2,3 and 5. These are at 97.67, 72.77 and 22.98 MHz. As m increases, these spurs seem to be diminished when lowering the TX power. The spur at 22.98, which has been reported by 3 others with similar strength, is the most problematic as it is essentially unfilterable. At power output of around 15 dBm, it appears that it will be at least 60 dBc, which exceeds regulations. Also, when the AD9866 Fs is raised in V2.0, this spur should increase in frequency and decrease in amplitude.

The most infamous spur is seen at 48.83 MHz. This is Fs - Ftx and I suspect is partly due to the quality of the interpolation filter in the AD9866. This remains at a constant 57 dBc even when varying the TX power. The worst case I measured was 53 dBc at TX frequency of 29.695 MHz. At that frequency, the spur is lower at 44.038 MHz. This too will improve with the higher v2.0 Fs by moving up another ~6 MHz and diminishing by ~3 dB.

MHz	20.4	20.1	17.4	14.4	11.5
22.98	-54.2	-55.0	-62.9
48.83	-56.8	-57.8	-57.1	-57.3	-57.6
72.77	-36.1	-37.8	-42.3	-46.9	-50.9
97.67	-47.6	-47.8	-48.0	-54.7	-55.6
98.62	-54.1	-54.5	-53.9	-53.6	-53.1
120.65	-60.7	-64.6
122.56	-17.2	-16.9	-17.5	-17.5	-17.6
147.46	-54.7	-52.5	-46.5
172.35	-39.9	-33.2	-38.6	-40.2	-41.1
197.25	-61.4	-62.4	-54.3
220.23	-60.2	-63.7	-59.6	-63.6
221.19		-61.9			-64.6
222.14	-56.6	-56.7	-59.0
245.12			-62.5
247.04			-64.4
270.02	-44.1	-45.7	-46.0	-45.5	-45.6
319.81	-50.3	-50.9	-54.0	-53.6	-54.0