Injection locking LNB LOs
This LNB is housed in a flat backed casting, so access to the
back of the pcb is very easy, just requiring a hole to be drilled
through the rear face of the casting, as the first picture show.
The nominal LO frequency is 9.75 GHz.
Here is a video (1.8 Mb) showing the LNB LO following an external oscillator.
Cambridge Industries AE88
The injection input can be brought out from the back of the pcb and through the main casting with this LNB also, as can be seen from the following pictures:
With this configuration, the
following results were obtained (nom 9.75 GHz):
LO frequency: 9.75 GHz. Injection applied to 50R
coupling line load resistor, as in previous examples
well as providing a modest gain, the LNBs internal puck oscillator
ought to add selectivity too. A simple single x23 frequency multiplier
was built, as below, to see if any selectivity was evident.
HSMS-8102 schottky pair (often used as an LNB mixer, and marked '2R')
is only rated at 75mW dissapation, but since no matching was used, the
drive was initially set at 100mW (well, 100mW into 50 ohms, that
is). Output from the pipe-cap filter was -27dBm, so 37dB of gain was
required to bring this up to 10mW. Two modified LNBs were used for this
provide better frequency stability, the HP8640 used in the previous
test was replaced with a crystal multiplier chain from an old M296 UHF
receiver. This runs x8 from a 53MHz 3rd overtone crystal. Output was
about 40mW, resulting in several mW from the x23 set-up above. This was
used for the set-up to provide the following pictures, which
shows the various LNB IF spectrums (LH three pictures) and x23
multiplier output spectrum.
injection locked LNB in this test was the Cambridge Industries AE88
comparison, the LH picture shows the IF output spectrum for a strong
3cm signal when only the internal un-locked LNB was used. Second left
shows the same IF otput, but this time with the LNB LO injection
locked. At first glance, the added spurious responses look
dissapointing, but removing the 3cm signal still results in IF output
port components (third LH pic). These are mainly the 424 MHz multiplier
drive leak-through, and its second harmonic, caused by poor screening
of the test set-up - see below:
Many of the IF spurious products are due to intermod between
the LO leak-through and the 3cm signal, and not the sidebands that are
a part of the x23 multiplier spectrum (RH pic of the four spectrum
pictures), so the LNBs internal LO oscillator has provided selectivity
- just a pity that the 424MHz leakage hides the effect.
Another factor to be considered is that the 616MHz IF is some
20dB down the slope of the LNBs IF high pass filter, with this
particular LNB. Some of the newer LNBs (ie, Thomson 13553) do not slope
the IF until about 300MHz, so would have been more suitable
(the IF frequency can't be varied that much, because it is determined
by the injection locked oscillators nominal puck controlled frequency
(+- a few hundred MHz), and the standard
'lo' oscillator for the UK market is 9750 Mhz).
Injection locking is worthy of consideration. The phase noise
of the resulting oscillator is much improved close in, and the far out
noise was good anyway - but no figures here - absolute value
measurements are for another day...