IC9700 part 2. Some numbers

I spent all afternoon re-testing and confirming measurements on the receiver sections of the IC9700.

 My measurement of noise figure are based on the Y factor method, using HP noise heads with known and well characterised ENR.

Noise power measurement at the receiver output used Moetronics Spectravue Continuum mode running on a Lenova desktop PC and SDR-IQ.

This Y factor technique is well known, although the use of the SDR-IQ to measure the resulting noise output is possibly a little unusual.

Like all these measurements, there are uncertainties in the measured values due to assumptions, rounding and the tolerance of out-of-cal test gear. The values given here are best efforts to get it right, but no guarantee is given as to accuracy.

Noise figure

Noise figure at 292k ambient.

144MHz 

Preamp on   preamp off

4.2dB          17.3db

432MHz

Preamp on   Preamp off

4.8dB          15.9dB

1296MHz

Preamp on   Preamp off

4.8dB          8.7dB

A low ENR HP346A was used for the preamp on and an HP346C for the preamp off  (higher ENR required for 144 and 432MHz)

High noise figure is expected with the direct sampling technique, without preamplification, as used in the IC9700 

The 1296MHz down conversion arrangement loses some of the direct sampling advantage but does allow a lower noise figure. The preamp is still worth switching in, however, but does not have such a profound effect as on the lower two bands.

On-air listening tests and signal generator tests seem to confirm that these numbers are not far out. 144MHz, without the preamp switched in, is particularly deaf, but quite acceptable with preamp on.

Spurious free dynamic range

144MHz only so far

Two tone dynamic range

Using two very low phase noise test oscillators

The two oscillators are combined in a high isolation combiner and then well attenuated.

Applied at the receiver input, the levels were adjusted to give a third order product at the receiver noise floor.

The test was done with the preamp on and repeated with it off.

The IP+ was then used to test its effect.

Preamp on

89dB at 20kHz tone spacing. I don’t have any low phase noise oscillators at 2kHz spacing

Frequencies 144.100 and 144.120MHz 

Preamp off

91.7dB at 20kHz tone spacing. Same test frequencies. 

I found that IP+ was only effective with preamp on. The dynamic range was not changed, only shifted upwards. However, the noise floor increased by 3dB. This was as expected.

This form of dynamic range test, with a direct sampling SDR receiver, can give misleading results as an SDR does not follow the usual rules for third order intermodulation. It is possible that the third order product may approach the noise floor level at some lower levels of two tome input, dropping away again as the tone levels are increased. The quoted levels were found without difficulty but I cannot be sure that one or more lower levels may have been missed. IP+ helps to overcome this problem by introducing noise as a ‘dither’ signal. As I don’t have the IC9700 service manual and schematics I cannot be sure this is the technique that ICOM used. The 3dB shift of dynamic range (only a shift of position, not an increase in dynamic range) when IP+ is on suggests that the  -89dB figure is not that far off.

S meter calibration

As you would expect of an SDR based receiver the S meter calibration is fairly good. 

Each S point is 3dB and above S9 each 10dB measures 10dB!

Preamp on 

S9 = -95dBm

Preamp off

S9 = -84dBm

If  an 11dB (approximate) gain preamp is used on 144MHz then  if the noise figure of that preamp was about 0.8dB, the overall preamp on and off noise figures are about right.

I still need to test on 432MHz and 1296MHz

These measurements are my copyright. If you want to quote them, please acknowledge this blog.

I offer no guarantee as to the accuracy of the measurements. They are best effort and may be amended following further testing.