Almost All Digital Electronics (AADE) 10MHz frequency standard review

I've purchased this 10MHz frequency standard to assist me in aligning an amateur radio transceiver kit that I plan to build in the coming months.

This device does a simple job: produce a stable and accurate known RF output. I purchased this for $55 (approximately £35), plus postage and handling from Milestone Technologies (aka Marshall Emm N1FN, owner of Oak Hills Research and other ham radio-oriented businesses in Aurora, Colorado, USA). AADE is a separate business run by Neil Hecht and Milestone Technologies is a reseller of some of their products.

The AADE RF standard is a TCXO operating from a regulated 5V supply (via an 7805-type regulator) and producing a 10MHz, with a cascade of decade dividers  (i.e. IC chips that divide the frequency by 10) to allow switchable outputs of 10MHz, 1Mhz, 100KHz, 10KHz, 1KHz, 100Hz and 10Hz and 1Hz outputs. The 5V signal is passed to the BNC female output plug via a 51Ohm-510Ohm voltage divider to yield a 0.5Vpp square wave signal.

The box was well packed with styrofoam 'peanuts', coming with the RF standard (ready-made), hard plastic box, some fixing self-taping screws to secure the top plate to the box and a 1-page instruction sheet, which includes a circuit diagram.

The instructions explain that the connected 9V (PP3) battery connector is part of the testing and alignment process. The RF standard is not intended for battery operation and owners should supply their own wall wart DC or AC power supplies, soldering the cables to the PCB board marked - and +. The RF standard can use 6-18V DC or 6-12V AC.

I desoldered the 9V battery connector and reused a car boot sale-salvaged 12V DC 1.5A wall wart (UK plug type) to good effect. [Yes, I tested the wall wart to check it worked and validate the output of 12.28V DC, within 2.5% of spec].

 To do this, I drilled the case to create one hole to allow the cable into the case, using a 12mm cable gland to tighten down on the cable and prevent any pulled cable from tugging on the soldered ends/PCB board. I also added an underwriters knot (i.e. electricians knot) to ensure tug-free use.

Once sealed up, I tried to measure the output. AADE states two things in the instruction sheet:

1. Examples of this device have been calibrated against rubidium standards to 0.1ppm accuracy (1Hz error at 10MHz).
2. Neil Hecht of AADE has no long term experience of how these products age. However, he does note that the same TCXO is used in a number of his frequency counters and drift is very low (he comments of one example recalibrated after only 7 Hz drift in 6 years).

With access to an oscilloscope, I did some quick tests to gauge how true the outputs were at various frequencies. My oscilloscope is not intended as a frequency counter itself, offering 50ppm error according to its specifications.

Below are some screenshots from the 'scope, showing RF standard outputs at 10MHz, 100KHz and 1Kz respectively. Frequency counted is shown below the waveforms and suggests the RF standard is 'close' to the target outputs.

You can see the square wave output is rather rough in output with ringing and harmonics distorting the shape. However, this device is not intended as a signal generator, but purely as a stable and accurate 'metronome' at RF frequencies.

My next test was with a ham radio rig: using my Elecraft KX3, I positioned the RF standard close to the antenna socket of the rig, but not connected (no coax, no wires).  I expected there to be enough power and proximity for the sensitive receiver section to 'hear' the RF standard output.

Using SSB on USB, I started just above 10MHz and listened in headphones to the beat frequency generated in the receiver between the rig's VFO setting (10,001,000 Hz) and the RF standard output (selecting 10Mhz on the switch). As the two frequencies differ by less and less, the difference becomes low enough to fall into audible range (e.g. 1000Hz, 800 Hz, etc...). By dialing the VFO closer to 10MHz on the rig, I heard the audio tone lower and lower until at 10,000,070Hz I heard nothing more, as the tone went too low for my ears.

I repeated this exercise in LSB and starting below the target, at say 9,999,000 Hz, I heard the audio tone descending as I increased the VFO until 9,999,930Hz when the tone was too low for my hearing.

This holistic analysis suggests the RF standard output for 10Mhz is indeed within 70Hz of target, i.e. 7ppm error. That is incredible for a simple circuit and matching low cost of £35! 

The low cost is apparent in the paper facia stuck on the box top rather than something made of plastic, while the PCB with through-hole plating is effective, but not far from home-made etching standards. My version appears to have some soldering iron touches to the DC supply ripple electrolytic capacitor's casing: a few melted contact areas. 

Overall, I do recommend this product as a simple and effective and apparently accurate (<10ppm) RF standard that is very portable (light and compact) and does one job quite well.