Injection locking LNB LOs


 
In some ways, this was simply an attempt to use the on-board ceramic puck to add further selectivity to the LO  of an external high stability multiplier chain, though it was also hoped that the internal oscillator could be locked to a sub-multiple of the LO frequency. The page is written in chronlogical order of measurement.

Initial set-up

An easy way to determine lock is to feed the LNB output into a spectrum analyser, and have a reasonably strong in-band signal running nearby. With the injection source switched off, the down converted signal is centred on the analyser display.  Switching on the injection source (I used a signal generator) and setting to +10dBm, swish the frequency around the nominal LO frequency. When out-of-lock, you see that not only does the main LNB IF signal not follow, but you can see additional IF components due to the mix of injection and signal frequencies and intermod products of these with the original IF. As the injection frequency approaches that of the internal LO, all the IF components converge to one point, and the internal LO should lock onto the injection frequency. At this point, as the injection frequency is moved around, the internal LO will follow, until at some point it drops out of lock.

Grundig AUN14S

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.
AUN14S  pcb Eudyna_LO

The pictures show the location of the drilled hole (red/white concentric circle near largest ceramic puck) and where it connects into the oscillator circuit


With this LNB, a mmic (Eudyna FMM5201MLT4E1) is used in the oscillator stage. It uses a single coupling track to the puck, as shown. A convenient place to inject an external locking signal is at the terminating resistor for this track.

The nominal LO frequency is 9.75 GHz.

LOCKING RANGE FOR COMBINATIONS OF INJECTION DRIVE LEVEL AND FREQUENCY (AUN14S)
Injection level
F.inj = LO
F.inj = LO/2
F.inj = LO/3
F.inj = LO/4
+10 dBm
4.26 MHz
0.18 MHz
no lock
no lock
0 dBm
1.09 MHz
no lock
no lock
no lock
-10 dBm
0.36 MHz
no lock
no lock
no lock
-20 dBm
0.1 MHz
no lock
no lock
no lock

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:

ae88 ae88_pcb ae88_LO

With this configuration, the following results were obtained (nom 9.75 GHz):

           inj-hi                                  inj-lo    

                                LNB LO too low in frequency                         response at near lock                         LNB LO too high in frequency

 

LOCKING RANGE FOR COMBINATIONS OF INJECTION DRIVE LEVEL AND FREQUENCY (AE88)
Injection level
F.inj = LO
F.inj = LO/2
F.inj = LO/3
F.inj = LO/4
+10 dBm
200 MHz +
4.85 MHz
0.57 MHz
0.63 MHz
0 dBm
23.1 MHz
0.354 MHz
0.03 MHz
no lock
-10 dBm
2.42 MHz
0.04 MHz
no lock
no lock
-20 dBm
1.02 MHz
no lock
no lock
no lock

    

EASS LNB

LO frequency: 9.75 GHz.  Injection applied to 50R coupling line load resistor, as in previous examples

EASS EASS1 EASS2

LOCKING RANGE FOR COMBINATIONS OF INJECTION DRIVE LEVEL AND FREQUENCY (EASS)
Injection level
F.inj = LO
F.inj = LO/2
F.inj = LO/3
F.inj = LO/4
+ 10 dBm
200 MHz +
0.58 MHz
0.44 MHz
0.52 MHz
0 dBm
7.65 MHz
no lock
no lock
no lock
-10 dBm
1.556 MHz
no lock
no lock
no lock
-20 dBm
0.336 MHz
no lock
no lock
no lock


Simple x23 multiplier

As 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.

x23

The 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 purpose.

To 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.

The injection locked LNB in this test was the Cambridge Industries AE88 detailed earlier.


 
intLO extLO extLO - no signal LO

        IF output - internal LO only          IF output - LO locked to ext multiplier          IF output - LO locked, no signal              External multiplier spectrum

For 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:

inj-lock-aasy  inj-lock-assy1

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).

Conclusions

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...