X-Lock Frequency Stabilization via the RIT circuit

By Bob Mutton VK2ZRM


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


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

The resulting DC level is applied to the  RIT input of the VFO. This output of the interface is centered at approximately +6.0 volts, superimposed on this is are the small "correction voltage" variations on the X-Lock which provide frequency compensation. When RIT operation is selected, the interface output voltage shifts according to the RIT control setting. The X-lock correction voltage remains superimposed upon the new RIT control voltage. Vref is set to be close to the gross DC level on the VAR output at switch on. In the case of the VFO-820, this will be close to +4.58V.


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.

Fig.1 Block Diagram

The block diagram below illustrates the operation of X-Lock and the interface and how they interact with the VFO.


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.

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.

Picture 1. Interface PCB and layout

Picture 2. Interface Installation

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.

Fig 2. Interface Schematic

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.


Bob Mutton, VK2ZRM