Frequency Level Mechanism
(MHz) (dB)
70.04 0 RF-LO (Desired response)
70.06 ~70 RF-LO (Baseband mirror image)
48.56 -75 LO-RF (Mirror image)
75.06 -120 RF-LO (RX10700 mirror image)
107.86 -107 2xLO-RF
129.34 -99 RF-2xLO
167.16 -117 3xLO-RF
188.64 -114 RF-3xLO
226.46 -104 4xLO-RF
247.94 -104 RF-4xLO
285.76 -106 5xLO-RF
307.24 -99 RF-5xLO
. . .
. . .
900.24 -88 RF-15xLO
Table 2.First order responses of the RX70 + RX10700 + Delta44
combination.
Level is dB below desired response at 70.04MHz.
The data of table 2 shows channel 1, the worst channel.
The stop band attenuation of the 70MHz filter could be greatly
improved by the addition of more screen walls that would have to
be screwed to the lid of the box.
Just removing the lid will improve the mirror image by 10dB and
taking the board out of the box improves by anothewr 10 dB.
Bearing in mind that the RX70 unit is an IF input, the first
order false responses should not affect system performance.
The converters preceeding the RX70 unit should have an IF
amplifier to make the mixer see a low noise figure.
This IF amplifier will reduce the first order spurs by at
least 30dB and with some selectivity at the antenna input
all first order spurs belonging to the 70MHz IF will be
insignificant.
The second order spurious response, the overtone of 35.020 MHz
generated by the RF amplifier has a level of -33dB at an input
level of 0dBm.
This signal drops twice as fast as the input signal (in the dB scale)
With a system noise figure of 15dB the noise floor is at -132dBm
in 500 Hz bandwidth so the second order spur is at the noise floor
at an input level of -49.5 dBm at 35MHz.
The corresponding second order intercept point is at +33dBm.
It is important that the converters used in front of the RX70
unit do not deliver more than -50dBm below 40MHz or so.
Second order intermodulation between signals, the sum frequency
of which the second harmonic is a special case will then be well
below the system noise floor.
Remember that the first amplifier at the antenna input will raise
the noise floor by at least 10 dB.
There are several other second order spurious responses between
35 and 100 MHz.
They are listed in table 3.
Frequency Level at 0dBm Mechanism
(MHz) (dB)
35.020 -33 LO-2xRF
53.93 -125 2xLO-2xRF
64.67 -132 2xRF-2xLO
83.58 -150 3xLO-2xRF
94.32 -146 2xRF-3xLO
71.295 -128 RF-LO (RX10700 response)
Table 3.Second order responses of the RX70 + RX10700 + Delta44
combination at 0dBm input power.
Level is dB below desired response at 70.04MHz.
At 0dBm input power the second order responses are close to the
noise floor in 500Hz bandwidth except for the very strong
second harmonic response at 35 MHz discussed above.
Only the RX10700 response, 2 x LO(13.275) - 2 x RF(11.995)
will survive the filters in the preceeding converter that
will produce the 70MHz signal but the level is low.
Table 4 shows the strongest higher order spurs in the 35 to
100 MHz range.
Frequency Level at 0dBm Mechanism
(MHz) (dB)
35.953 -156 2xLO-3xRF
70.973 -132 6xRF-7xLO
71.440 -102 5xLO-4xRF
71.058 -150 11xRF-13xLO
Table 4.The strongest higher order responses of the RX70 +
RX10700 + Delta44 combination at 0dBm input power.
Level is dB below desired response at 70.04MHz.
Spurious response data is not often published in receiver tests.
As a comparison, the IC706MKIIG was tested on 50 MHz.
With preamp off the noise figure is 9.5dB giving it a 5 dB lower
noise floor than the RX70 system.
The IC706 has 1dB compression at about -16 dBm.
Close to the 1 dB compression point the IC706 has a large
number of spurious responses but already at -30dBm it
has about half a dozen well audible responses within
+/- 1 MHz.
The strongest of them reaches the noise floor in 500Hz
bandwidth at an input level of -52dBm.
I do not know how better receivers perform but I do think
spurious responses is an important aspect of receiver performance.
Two signal dynamic range
A weak and a strong signal are combined in a 3dB hybrid and the result
is sent to channel 1 of the RX70 + RX10700 + RX2500 + Delta44 (mod) signal
processing chain.
The weak signal is placed at 70.040 MHz and a notch filter at the same
frequency is inserted between the strong signal and the hybrid.
The level of the strong signal is adjusted until the S/N of the weak
signal is degraded by 3dB.
Table 5 shows the result of this test with the Delta44 board in maximum
and in minimum gain mode.
Frequency Level Level
separation [+4dB] [-10dB]
(kHz) (dBm) (dBc/Hz) (dBm) (dBc/Hz)
-2000 27 186 27 189
-1000 21 180 20 182
-500 18 177 15 177
-300 15 174 13 175
-200 13 172 11 173
-150 13 172 10 172
-100 12 171 9 171
-80 7 166 7 169
-70 6 165 5 167
-60 8 167 8 170
-50 -13 146 -24 138
. . . . .
visible . . . .
passband -13 146 -24 138
-50 to +20 . . . .
. . . . .
20 -13 146 -24 138
30 2 161 -19 143
40 4 163 2 164
50 6 165 5 167
60 9 168 6 168
70 10 169 7 169
80 11 170 8 170
100 12 171 9 171
150 13 172 10 172
200 13 172 11 173
300 13 172 12 174
500 15 174 13 175
1000 17 176 15 177
2000 27 186 27 189
Table 5. Two signal dynamic range of a RX70 + RX10700 + RX2500 + Delta44
combination.
The Delta 44 is run at low gain [+4dB] and at high gain [-10dB]
An inspection of table 1 shows that the dynamic range is better by
8dB for offending signals within the visible passband if the Delta44
is operated in minimum gain mode.
One can also see that the dynamic range is not much better for
signals outside the visible passband if the Delta44 is operated in
minimum gain mode.
A better soundcard than the Delta44 will not improve performance much unless
the gain is reduced in the converters. Reducing the gain while using the
Delta44 is a bad idea because lower gain will not reduce the noise floor
much, the Delta 44 already contributes most of the noise floor.
Less gain in any of the converters will reduce the noise figure and bring
the noise floor closer to the point of saturation.
The parts of the RX70 + RX10700 + RX2500 + Delta44 system are matched to
each other to optimise the performance for a 500 kHz wide IF strip at 70 MHz.
At frequency separations above 100 kHz performance is limited by the
sideband noise generated by the RX70 unit.
At 2 MHz frequency offset, the system is limited by saturation.
As can be seen from the table 0.5W is required at the input for a
3dB loss of S/N for a very weak signal.
The noise floor then drops by about 0.5dB while the signal drops by 3.5dB.
Three signal dynamic range
For two signals within the visible passband, the third order intercept
point, IP3, is +17 to +23 dBm depending on the frequency separation.
For two signals outside the visible spectrum IP3 is higher.
Table 6 shows the level of the third order intermodulation signal at
different signal levels and frequency separations.
The received signal or intermodulation product is at 70.040 MHz
Input Signals IM3(30kHz) IM3(300kHz)
level S-meter S-meter S-meter
(dBm) (dB) (dB) (dB)
0 140 *** 82.2
-5 135 71.3 66.8
-10 130 59.0 51.8
-15 124.9 39.0 36.9
-20 120.0 25.6 22.0
-25 114.9 14.9 7.2
-30 110.0 1.5 -7.0
Table 4.
Signals and third order intermodulation products.
The levels are from the Linrad S-meter. Levels above 124.9 are extrapolated.
The third order intercept point can be obtained from the data in
table 4:
IP3(30kHz) = +26dBm
IP3(300kHz) = +29dBm
Compared to the RX10700 + RX2500 + Delta44 combination, adding the
RX70 converter degrades the third order intercept point by 7dB while
the noise figure is degraded by 1 dB.
The total loss of intermodulation free dynamic range is thus 8 dB.
By replacing the J310 FETs with a GaAs FETs it would be possible
to improve the mixer IP3 from about 31dBm to about 40dBm according to
i.e. U.Rohde QEX Jan/Feb 2003 p 27.
I have made a few experiments with MGF1302 GaAs FETs but they were
not encouraging.
With IP3 = 29dBm at a noise figure of 15dB the IM3 free dynamic
range in 500 Hz bandwidth is 107 dB.
This is good enough for my needs, the J310 mixer is just about good
enough to match the sideband noise of the crystal oscillators.
Improving the RX70 mixer will not make any significant improvement
to the two signal dynamic range, table 5 above, so I see no reason
to replace the J310 transistors.