RAN Technology

Spectral Purity using Epson Programmable Oscillators

Crystal Replacement 0 Comments 03/12/2019 

Do harmonics matter if there's no way for them to be heard?

Posted By: Robert Nickels (ranickels)


In the 5 or so years I've been using the Epson oscillators I've looked at the spectral output many times.   Without a doubt the harmonic output from the Epson oscillator is high as would be expected  from a square wave device.   The question to me has always been "does it matter"?

PLL controlled digital oscillators have long been used as LOs in receivers and transceivers, including the Si570 and Si5351 which also have square wave outputs with high harmonic content, so I felt the Epson oscillators should be no different, and that has been my experience.    I've also used the clock generation capability of the Raspberry Pi extensively for wspr, which is also a square wave signal that is nicely filtered by use of a  7 pole low-pass filter taken straight from the GQRP harmonic filter guide.    I've also designed my own lowpass and bandpass filters for wspr using Elsie which  work equally well with spots from around the world using 40 milliwatts of power.

All of which is to say - if you want a clean output directly from the oscillator you will need a filter to remove harmonics.   However my primary interest in using the Epson oscillators is for crystal replacement in radios where they either provide injection to a mixer or drive amplifier stages, all of which already have tuned circuits that are resonant at the frequency of interest.   I've done the comparison vs quartz crystals numerous times and the answer has always come up the same:   the spectral purity using the Epson oscillator is no different from using a crystal.

That said, we all know that merely using a crystal guarantees the absence of harmonic content (which is why hams have used low pass filters and sometimes received pink tickets!)    But my point is that the harmonic attenuation provide by the transmitter is the same with a crystal or an Epson - it's a function of the transmitter itself.    Many hams use antenna tuners nowadays which provides additional high selectivity to prevent whatever harmonics may be present from radiating far.

Today, FCC part 97.307 requires 43dB attenuation of HF harmonics for radios built since 2003,  40dB for those built between 1978 and 2003 (30dB for < 5 watt rigs), and those built prior to 1978 are exempt.   Even though crystal replacement in boatanchors is thus exempt from 97.307, it's always been my goal to at least come  close or if possible to meet the FCC regulation, or at least to be no worse than the same radio is using crystal control.   Attached are three comparisons of  Epson oscillators showing the spectral output for various configurations:

1.   Raw Epson vs. G-QRP LPF.    This shows the attenuation provided by the 7 pole 20 meter LPF per the attached design guide.   While the harmonics are still present, they are attenuated by around 40dB.

2.   HW-16 - comparison showing there is no difference in harmonic content whether a crystal or an Epson oscillator is used (this is with the Epson oscillator driving a one transistor broadband amplifier of my own design).    The second harmonic is down about 38dB and the third is actually better than the crystal.

3.  Bendix Skipper 101 Marine radio.  The "crystal" spectrum is the normal  2-tube transmitter using a a 12AQ5 oscillator driving a 12JB6 PA for about 20 watts output.   The second plot shows the output of the untuned amplifier connected directly to the grid of the PA tube, bypassing the crystal oscillator stage in he the radio entirely  (it is also untuned).   No difference in harmonic output can be seen with both providing about 36dB attenuation of the second harmonic.

4.  My experience says that tuned circuits in radios are highly effective at removing harmonic content when Epson oscillators are used to drive tube type transmitters and all kinds of mixers.   A parallel resonant tank using a 2.2uH inductor and combination of fixed and variable capacitor was adjusted to resonate at the 13.85MHz frequency and coupled it to the output of my untuned amplifier.    A two-turn link enabled the spectrum analyzer to create the "tank" plot which shows about 30dB attenuation.  (I would also note that this was a "lash-up" of two existing boards using short leads but not optimal RF wiring and grounding practice and a properly designed tuned RF amplifier could no doubt be even better).

We use class C RF amplifiers because they provide high efficiency, but at the cost of high harmonic distortion.  That's why they are invariably used with tuned loads that are resonant at the frequency of interest.   So even if we fed a perfect sine wave into a class C amplifier, it will create it's own distortion that must be filtered out.    To my way of thinking this is why none of the transmitters I've tried Epson oscillators with cares whether the input waveform is sine or square -  the class C PA is biased such that it is conducting less than half the time and relies on the flywheel effect of the resonant tank to keep the sine wave going during the non-conducting portion of the cycle.

If you use the Epson oscillators in a stand-alone mode or with an amplifier, I strongly suggest using either a LPF or tuned output circuit in the amplifier - the data presented here offers a guide as to the degree of spectral purity that can be obtained using LPF and tuned circuits.   But for general crystal replacement, it is clear that the harmonics present in the square wave output have no more impact on harmonic generation from a typical transmitter than when using a quartz crystal for frequency control.

Description Comment  
G-QRP harmonic filters by G3RJV


Mystery Transmitter

where's the copper subchassis from?
Category: Vintage Ham Radio
 I didn't set out to buy this homebrew CW transmitter, it was included in an auction lot that I wanted so I had to take it to get the desired item.   But even though I always like to see good quality homebrew gear, this one is just a mystery.Most of the transmitter, including 6CL6 oscillator and buffer stages and a 6146 PA are built on a sub-chassis that is clearly part of a com...  READ MORE
- Robert Nickels (ranickels),  07/15/2021 

The Vector VR-50

last gasp from Swan/Atlas founder Herb Johnson
Category: Vintage Ham Radio
 Most hams are aware that Herb Johnson W6QKI founded Swan in Benson Arizona to make single-band SSB transceivers and then moved his operation to Oceanside CA where Swan thrived throughout the 1960s.   Swan merged with Cubic Corporation in 1967, and Johnson managed Swan as its subsidiary until 1973. Johnson founded Atlas Radio in 1974, with the assistance of Southcom International fo...  READ MORE
- Robert Nickels (ranickels),  07/12/2021 
   The Harvey-Wells Company was formed through a partnership between Clifford Harvey W1RF, and John Wells W1ZD in 1939.   Cliff Harvey had earlier founded Harvey Radio Labs in 1933, and prior to that, he was associated with the Hendricks and Harvey Company, another partnership. Producing police radios, transceivers, transmitters, and crystals. Their most popular product was the TBS-50 trans...  READ MORE
- Robert Nickels (ranickels),  06/18/2021 
    I became aware of a few hams playing with a thing designed to receive digital TV in Europe on your laptop - a little plug-in dongle that used an RTL-2832 IC and sold for $20 or so.    Some clever fellows had determined that it could be put into "radio mode" in which it would generate an IQ stream over USB, and by writing to control registers in the tuner IC, it could...  READ MORE
- Robert Nickels (ranickels),  06/13/2021 
    A while back I acquired a six channel HF transceiver made by the Radio Industries division of Hallicrafters, probably in the 1960s, called an SBT-20.    It is capable of 20 watts SSB or 5 watts AM (or CW with an optional board) in the range of 2-12 MHz and thus was probably aimed at commercial and light-duty military applications.   The radio could by ordered with fu...  READ MORE
- Robert Nickels (ranickels),  06/07/2021 
    I'm always thinking about interesting combinations of equipment to try out on the air.   One day while rearranging the shack I was getting ready to connect my Globe Scout Deluxe back up with a Collins 51S-1 receiver when the LED (the energy-efficient version of the old light bulb) came on.When I first started playing with DVB-T dongles back in 2012 I wanted an upconverter so I ...  READ MORE
- Robert Nickels (ranickels),  05/29/2021 

6AG7-6LG Novice Transmitter

Classic circuit with some modern twists
Category: Vintage Ham Radio
 There's a good chance that more homebrew ham transmitters have been built using a 6L6 than any other tube, and when combined with the superior performance of the 6AG7 oscillator, it's a hard combination to beat (click here for an explanation of the 6AG7's benefits)I'll be adding more info about this project soon, but one of my goals was to reproduce what was a budget-friendly...  READ MORE
- Robert Nickels (ranickels),  05/21/2021 
    All who operate AM in the midwest know and probably have talked to Masa, AB9MQ, who is a very active AM operator.    Having become interested in ham radio while still living in Japan in the early 1960s,  Masa's memories of the "dream rigs" is a bit differen than most US hams, and because the markets were still quite regional at the time, much of the ham gear...  READ MORE
- Robert Nickels (ranickels),  05/18/2021 

Hudson American Corporation

Manufacturer of marine radios in the 1940s
Category: Vintage Radio
 I enjoy playing with old marine radios that operated in the AM mode between 2-3 MHz.    This was the standard for "ship to shore" radio and telephone service from marine radio operators from after WWII until about 1970 when SSB was phased in and AM became obsolete.     Through this era a number of manufacturers were major players including RCA (Ra...  READ MORE
- Robert Nickels (ranickels),  05/02/2021 
    I've always thought that knurled aluminum knobs were a high-class option for radio gear, and while they are more durable than plastic they do accumulate tarnish, corrosion, and grunge from dirty fingers over the years and start to look poor.   Fortunately it is easy to restore them to a new attractive appearance using a bead blaster. Mine is a Harbor Freight floor-standing...  READ MORE
- Robert Nickels (ranickels),  04/29/2021 

VIEW News Item

Announcing the W9DYV Radio Society

honoring SSB pioneer Wes Schum W9DYV
VIEW News Item

Hamfests Return!

A quick look at the Wauseon OH swap on June 6, 2020
VIEW News Item

Making A Transistor Radio

in memory of Rev. George Dobbs G3RJV (SK)
VIEW News Item

Soviet Spy Radio found in forest

(oh yeah, I put that there...I'll take it now thanks!)
VIEW News Item

Take the Boatanchor Survey!

Let authors and organizers know what you're interested in
VIEW News Item

W9RAN's "VERSA-TR" (as featured in Dec. 2018 QST)

A Versatile T/R solution for SDRs and vintage radios
VIEW News Item


Cool mics from my collection and ones I wish were

(There are currently no Blogs.)