X-Lock
Frequency
Stabilization via
the RIT
circuit Foreword This article describes an interface designed and built by Bob VK2ZRM which allows the X-Lock to be interfaced to a transceiver via the RIT (Receiver Incremental Tuning) circuit. With this approach the X-Lock frequency correction voltage is summed with the RIT voltage to provide correction tuning as well as normal RIT operation. Whilst this technique requires a small amount of additional circuitry, it offers a significant advantage in that the additional correction varactor is not required. The interface has been tested on in a VFO-820, in principle it should work with all RIT/Clarifier schemes. Bob's original description of installing the X-Lock into a TS-520 can be read here. Ron Taylor G4GXO Introduction Recently I integrated an X-Lock VFO Stabilizer from Cumbria Designs into
my Kenwood TS-520 “boatanchor” rig.The X-Lock
kit exceeded my expectations
and provided “PLL” type stability to this old rig, which is fitted
with a free running VFO. This installation
of the X-Lock involved opening up the Kenwood VFO enclosure to fit
a varicap diode (a red LED), into the VFO tank circuit. This additional
varicap tunes the VFO over a range of a few kHz. The X-Lock correction
voltage is applied to the varicap to compensate for VFO drift. The limited tuning range of the compensation varicap offers a degree of
control similar to that of the existing RIT line. This got me thinking
about the possibility of installing X-lock without the need to open up the
Kenwood VFO unit by utilizing the RIT (Receive Incremental Tuning)
varicap for both RIT and drift compensation under X-Lock control. The
result is this project which describes my “X-Lock Interface”. The concept of the interface is very simple. The interface mixes three DC
signal levels, with the output applied to the VFO “RIT” input. The
three signal levels are: the X-Lock frequency correction voltage
“VAR”, minus a reference voltage, plus the RIT voltage. The formula
is: Interface output = (VAR – Vref) + RIT Only a single wire needs be changed in the VFO circuit; the RIT voltage line. The remaining connections are the input to the X-Lock which is taken from the VFO output, and the regulated +9.0 volt DC supply voltage from the VFO-820 VFO supply which is used to power X-Lock and the Interface. (See my previous article regarding using the +9v supply to power the X-Lock). It is important that the same regulated +9.0 volt source is used to power the VFO, the X-Lock and the Interface. |
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Fig.1 Block Diagram The block diagram below illustrates the operation of X-Lock and the
interface and how they interact with the VFO.
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The interface enables X-lock to stabilize the VFO output frequency
through the RIT line, while retaining the VFO RIT control and the X-Lock
“RIT memory feature. There seems to be no downside to this project other
than the added complexity of some additional circuitry. |
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Photos and schematic The two photos below illustrate the interface PCB and its installation inside a Kenwood VFO-820 external VFO. The X-lock is the lower PCB with the interface PCB mounted above it. To avoid drilling additional holes in the VFO cabinet, a sub-chassis is used which is mounted onto the rear of the VFO housing by utilizing the screw holes used to mount the serial number plate. |
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Picture 1. Interface PCB and layout |
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Picture 2. Interface Installation |
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The schematic below illustrates the circuit and typical control signal
ranges. The lower op-amp of the LM358 subtracts Vref from the X-Lock
output, leaving only the small control variations. The upper op-amp adds
this control signal to the RIT voltage from the RIT control. The output of
the op-amp goes to the RIT input of the VFO, thus stabilizing the VFO
output frequency. |
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Fig 2. Interface Schematic |
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The interface is powered from the regulated +9 volt supply from the Kenwood transceiver. The 7662 IC is a charge pump which generates a negative voltage so that the op-amp is powered from a balanced power supply. |
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Bob Mutton, VK2ZRM
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