GOOIT Gy550 frequency counter's lower frequency limit and operating beyond 1 minute

This is a work in progress!

Created: May 2013

How low can the counter read?

The GOOIT Gy550 frequency counter has an advertised requency range of 50MHz-2.4GHz.  A user's review of this counter also appears to confirm the inability to function below 50MHz. Although the counter is very attractively priced at <USD18, the 50MHz lower frequency detracts from its usefulness. However, it doesn't make sense why the lower frequency limit should be so uncharacteristically high.  To find out if specified limit represents reality, we tested one unit with a Kenwood SG-5115 synthesized signal generator.  The signal generator's output is connected to the counter's fully retracted telescopic aerial via a coaxial cable that is terminated at one end with crocodile clips (fig. 1). To complete the ground path, the coax's shield is connected to a brass standoff mounted on the PCB ground-plane. Using a -13dBm (0.05 mW) source power, the source frequency is progressively lowered beginning from 50MHz. The counter counted down reliably to 7 MHz and even displayed a full signal bar. However, the counter become erratic below 7MHz.. There also a  progressive reduction of the signal bar until it completely disappears at 5MHz.

Fig. 1 The counter can operate down to 7MHz when the source is directly connected to the counter's retracted telescopic aerial

To find out what causes the counter to fail below 7MHz, we studied the circuit posted by the aforementioned reviewer. Neither the uPC1667G pre-amp nor the MB506 prescaler have a lower frequency limit according to their datasheets. Our guess is that the lower frequency limit is most likely imposed by small-value coupling capacitors between the uPC1667G pre-amp and the MB506 prescaler (fig. 2). To verify this hypothesis, we plan to connect the source directly to the prescaler input. If this hypothesis is later confirmed, then the solution is to change to large value caps.

Fig. 2 The lower frequency limit is likely imposed by the coupling caps btween the uPC1667G pre-amp and the MB506 prescaler (Circuit from m1kta-qrp.blogspot.com/2012/06/gy560-frequency-counter.html)

Operating beyond 1 minute

Aside from the frequency limitation, another issue is the counter powering off automatically after 1 minute. While the 'auto power off' function is suitable for the target market - determining walkie-talkie's transmit frequency, it makes the counter unsuitable for applications requiring continuous monitoring. According to the ebay vendor, "haruyr_kb" the 'auto power off' cannot be disabled. But we discovered that the counter can operate continuously in the 200 kHz step mode.

Other wishes

Additionally, we also wish for the counter to be available with either 455kHz or 10.7 MHz offset for use as receivers' frequency display. Since the casing, telescopic aerial, brass stand-offs and battery are not needed when the counter is to be incorporated into another equipment, hopefully a 'PCB-only' version can be made available at an even lower price.  

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