Monday, 19 September 2016

Direct Sampling vs RTL-SDR with Upconverter vs SDRPlay for HF

Let's hear the difference between a generic chinese RTL-SDR dongle modded in 5 minutes, an RTL-SDR dongle with upconverter, and an SDRPlay on frequencies below 30 MHz with 20 foot of wire.
Background information and testing notes in the Manifesto, underlined text are links, bring you to a new page in a new window. Click / tap images for full-screen glory.


In the video



Direct sampling modded generic chinese RTL-SDR dongle: $8 from eBay, push a wire through a hole (detailed picture guide here link), connected to an outdoor wire.
Premium RTL-SDR with Upconverter: Nooelec SMArt (review link) with Ham-It-Up v 1.3 Upconverter (review link) in a metal case, yours for ~80 dollars from Nooelec (manufacturer link).
SDRPlay: All-in-one DC to daylight software defined radio receiver. $150 or thereabouts (review link, manufacturer link).


Alternatives



The AirSpy platform consists of a receiver and an upconverter. Can't comment on performance as I don't have one. Reviews praise excellent performance (such as this one, link).
Chinese all-in-ones: most of them are rip-offs, direct conversion receivers in nice wrapping (post how to avoid them link). Some of them feature an upconverter for 40-50 dollars, not tested so can't comment - I won't spend 50 dollars on a product with 30 day warranty when I've seen what's inside a chinese dongle (see images here, link).


Software choices and testing methodology



SDRUno for SDRPlay, SDRSharp for RTL-SDR with upconverter and direct sampling, because:
1) beginner RTL-SDR users will likely start with SDRSharp, so a familiar software environment can be presented,
2) two separate programs let me to use SDRPlay's dedicated software on one screen, and SDRSharp for RTL-SDR based dongles on other screen,
3) SDRUno requires a restart when removing a dongle, and I'm not immortal.
Hunting for signals: Find a station with SDRPlay, adjust gain and LNA for best audible audio, record, find same with RTL-SDR based setup, adjust settings for best audible audio, record, find same with a direct converter, record. Or the other way round. Switching between receivers took less than a minute.
I'll be the first to admit that I'm nowhere near proficient with either SDRSharp or SDRUno; nobody is, save the guys who's written the software. I simply adjusted available settings to enjoy a particular broadcasting station, then recorded results.
Broadcasting stations only due to availability, plus moral issues with ham SSB or CW conversations.
Personal moaning: how much I hate SDRSharp, especially after this: constantly crashes. Wanted to record more, but given up after two days and the umpteenth computer restart. SDRUno was stable as a rock.


Testing setup



Two 20 foot / 6.5m multi-stranded 1mm diameter copper wires strung from a first-floor window.
SDRPlay and RTL-SDR with upconverter sharing the same antenna with the SMArt's supplied antenna cable and mount, antenna wire wrapped around base screw.
Wire for direct conversion dongle wrapped around RG6 coax center conductor on one end, other end of center conductor goes directly into direct sampling RTL-SDR dongle.
Above antenna configuration takes less time to sip a cup of tea: throw wire out from a window, wrap antenna wire around center conductor, connect other end to receiver. Done.
Yes, grounding, capacitors, antenna isolation, better antenna to receiver connection, metal shielding, devoting time to software intricacies, a dedicated antenna tuner, an LNA4HF, bandpass filters, soldering, living in the Australian Outback, a T2FD, inviting NASA radio engineers over to dinner and listening to suggestions, a better sunspot cycle, plus countless other variables will undoubtedly increase receive performance.


The video



First: Direct sampling modded generic chinese dongle (frequency in SDR#), then
Second: Nooelec SMArt with Ham-It-Up v1.3 versus (125+frequency in SDR#), then
Third: SDRPlay (frequency in SDRuno).
Repeat.
Same stations, displayed frequencies are off due to drift and no TCXO in generic dongle and SDRPlay.



If doesn't play here, copy-paste the following YouTube link:

https://www.youtube.com/watch?v=g7qa8nQbIPge


Conclusion



Direct sampling works to an extent, such as a <insert crappy automobile name here> is better than walking. Inserting a wire into an RTL-SDR dongle can be done in a few minutes, and provide an opportunity to listen to stations below 30 MHz.
Using an upconverter, such as the Ham-It-Up v1.3 here, is better in most cases, but an upconverter costs around $50.
The SDRPlay is a great receiver, however, it costs $150 - you get what you pay for.

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