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RD JDS6600 Signal Generator: Warmup Time

An RD JDS6600 Signal Generator recently arrived from around the curve of the horizon, leading me to measure its warmup time:

RDS6600 Signal Generator - Warmup plot
RDS6600 Signal Generator – Warmup plot

Looks like it’s good to go after maybe 90 minutes and, after much longer, it settles to 10 MHz +36 Hz, for a correction factor of 0.9999964 on those days when you’re being really fussy.

The need for frequencies accurate to better than 4 ppm doesn’t happen very often around here, but it’s best to be prepared. It’s amazing what you can get for under $100 these days …

I measured the frequency by zero-beating against the Z3801 GPS Frequency Standard (purple trace in the middle):

RDS6600 Signal Generator vs. Z3801 GPS Frequency Standard
RDS6600 Signal Generator vs. Z3801 GPS Frequency Standard

Basically, trigger the scope on either trace, crank the JDS6600 frequency in 1 Hz, then 0.1 Hz steps, until the traces stop crawling past each other, and you’re done.

It’s worth noting you (well, I) must crank eleven 0.01 Hz steps to change the output frequency by about 0.1 Hz around 10 MHz, suggesting the actual frequency steps are on the order of 0.1 Hz, no matter what the display resolution may lead you to think.

The RDS6600 main PCB (Rev 15) sports a 24 MHz oscillator close to the Lattice FPGA:

RDS6600 Signal Generator - clock oscillator
RDS6600 Signal Generator – clock oscillator

The AD9850 step size worked out to 0.0291 Hz for the LF crystal tester. A 24 MHz clock would produce a 5.7 mHz step size, but that’s obviously no what’s going on. More study is indicated.

The bottom trace is the scope’s internal function generator, also set to 10 MHz. Zero-beating the JDS6600 against the scope’s output produces a similar result:

IMG_20190312_130925 - RDS6600 vs SDS2304X frequencies
IMG_20190312_130925 – RDS6600 vs SDS2304X frequencies

The scope’s function generator actually runs at (9.999964 MHz) × (0.9999964) = 9.999928 MHz, a whopping 72 ppm low. The on-screen frequency measurements don’t have enough resolution to show the offset, nor to zero-beat it with the Z3801 input, so it’s as good as it needs to be.

The Z3801’s double-oven oscillator takes a few days to settle from a cold start, so this wasn’t an impulsive measurement. Having the power drop midway through the process didn’t help, either, but it’s March in the Northeast and one gets occasional blizzards with no additional charge.

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  1. #1 by Raj on 2019-03-19 - 07:51

    I feed a GPS base 10Mhz and Osc under test like Rubidium unit to a SBL-1 equivalent and the IF out to a center zero analog (big) meter and try to get them synced with minimum drift of the needle.