17 July 2023

Motorola K1516AE VC-TCXO

With a very likely correct pinout at hand (see previous post) for the Motorola K1516AE VC-TCXO, it was time to fire it up. The pinout was correct indeed and I got a 10.000 MHz signal.

Testing a Motorola K1516AE TCXO on a breadboard.
Motorola K1516AE TCXO on a breadboard.

 What I noticed:

  • after 60 minutes at 27°C ambient temperature the case does not get sensibly warm
  • the voltage control of the oscillator gives about 200 Hz freedom
  • Vcc influences the generated frequency

In order to get a well behaved reference oscillator care must be taken to provide a very constant voltage supply and, probably, the desired termination impedance (or at least "constant"). Since I was using the bench power supply and a sub-optimal assembly I will not mention the 1 Hz drift over one hour of operation :)

The thought of building a reference oscillator spins up the need for a very stable source of 12 V (or thereabout as long as it is stable over time). Living on the time-nuts edge...

14 July 2023

Motorola K1516AA K1516AE TCXO pinout

I found a canned oscillator hanging around my desk. It is a TCXO or even VC-TCXO marked Motorola K1516AE  1813-0535 10000000 Hz. No pinout. I haven't found a datasheet online, or I used the wrong search keywords.

On the other hand with some keywords I get directed to an eBay item of a replacement board for an HP spectrum analyzer, the HP 8562A. Usually old manuals show the schematic diagram and luckily the whole PDF is available on Keysight website. It is 462 pages long with only block diagrams. But I was VERY lucky since at page 6-25 (217 in the PDF), figure 6-7, in a "manual change" section the oscillator diagram is shown in clear! And the K1516AA TCXO pinout is there!


Page 6-25 of HP 8562A spectrum analyzer manual, power supply, TCXO and TTL level generator.
Page 6-25, power supply, TCXO and TTL level generator.

Zooming in for improved readability (click on the picture for a larger version):

Zoom on the schematic depicting the TCXO part
Zoom on the TCXO part.

I have a K1516AE but I think the pinout shown for K1516AA will not be too far away.



02 July 2023

MEMS microphone with TDA1308 amplifier board - 2

This is part 2 of my work on this module. Read part 1 here.

In order to get a usable configuration of the MEMS module described in the previous post, I wired it and started playing with resistors while listening to myself on a pair of headphones since there is enough signal to drive them.

The circuit as-is is so sensitive that it is like wearing a hearing aid!

First of all, it works from 2.5V and up. I noticed no difference in the output quality or volume at higher voltage: the final setup will need to keep the voltage within specifications, i.e. below 4V.

To reduce the overall gain R5 needs to become smaller. That's easy since we can parallel another resistor.

To reduce the microphone sensitivity I need an higher R6, so the original 0 ohm must be removed.

I chose to use 10k trimpot to speed up the process of finding a suitable configuration and started with R6. Rather than desoldering I cut it away. While increasing R6 has some effect on the sensitivity, the circuit starts humming when begins a noticeable reduction in the sound picked up.

I chose to keep R6 at zero ohm for now and moved on to R5. In this case the trimpot is in parallel to the existing 222 (22k) resistor since we need a lower resistance to decrease the gain. When the gain is adjusted it's like acting on a volume control, so both voice and hiss decrease of the same amount.

Fine, time to act again on the microphone gain R6. This time I kept leads short and fit first 330 ohm then 5600 ohm. In both cases I could notice the decrease in sensitivity but the noise/hiss was unchanged.

I will not spend more time debugging this module. It does work, it is very sensitive, but it is noisy. The noise might come from the op-amp or the MEMS module itself. Regardless, this circuit cannot be satisfactorily used with a transmitter. Maybe a different batch could yield different results, who knows.

Other uses? Probably recording sound that is then post-processed to remove the hiss is possible. I leave the comments open (moderated) so that readers can share their stories with this module.