What sample rate to use in the GNU Radio SDRplay RSPDuo block for 1420MHz radio astronomy data collection?
SDR Play RSPduo – in gr-sdrplay3 module in GNU Radio it asks me to put in separate sample rate in options separate to center frequency. For 1420 MHz what sample rate should I use?
Good question — this is an important (and slightly confusing) detail of the gr-sdrplay3 GNU Radio block for the RSPduo.
👉 Key point:
The center frequency and the sample rate are independent parameters.
- Center frequency = the RF frequency you’re tuning to (e.g. 1420.405 MHz for the H I line).
- Sample rate = the rate at which the SDR digitizes the RF signal and delivers IQ samples to GNU Radio.
The sample rate you choose determines the width of the usable bandwidth around your center frequency.
✅ Recommended sample rates for RSPduo (single tuner mode)
The RSPduo supports only a set of discrete sample rates. Common stable ones are:
| Mode | Typical sample rates (Hz) | Usable bandwidth (Hz) |
|---|---|---|
| Normal (Single) | 2 000 000, 4 000 000, 6 000 000, 8 000 000 | ≈ 1.536–8 MHz depending |
| Dual tuner coherent | 2 000 000, 6 000 000 | ≈ 1.536 or 6 MHz |
| Low sample mode | 250 000, 500 000, 1000000 | Narrowband (0.2–1 MHz) |
💡 Tip:
For 21 cm line (1420 MHz) work, you usually don’t need massive bandwidth. A few hundred kHz to 2 MHz is plenty, because:
- The Milky Way H I line spans roughly ±200 kHz around 1420.405 MHz.
- Too large a bandwidth just wastes disk space and processing time.
⚙️ Suggested settings for 1420 MHz
- Center frequency:
1420.405e6Hz - Sample rate:
2.0e6Hz (2 MHz) — this is a good balance of resolution, stability, and data rate.- If you want narrower bandwidth and lower data rate,
250000or500000Hz is also fine. - If you are using dual coherent mode for interferometry, use
2.0e6or6.0e6Hz (these are the stable supported rates).
- If you want narrower bandwidth and lower data rate,
🧠 Note: If you pick a non-supported sample rate, GNU Radio will error out or silently round to the nearest supported value. Stick to the official rates above.
🛰 Example GNU Radio settings:
Center frequency: 1420.405e6
Sample rate: 2.0e6
IF bandwidth: 1.536e6 (optional — matches sample rate band)
AGC: Off (manual gain recommended for astronomy)
Gain: ~50 dB (adjust to avoid clipping)
If you’re doing spectral line observation, it’s also good to follow the FFT size = SampleRate / desired channel width rule when setting up your FFT sink or spectral averaging.
✅ Bottom line:
For most 21 cm spectral work on an RSPduo, set:
Center frequency = 1420.405 MHz
Sample rate = 2.0 MHz
and adjust FFT size + integration in downstream blocks for your desired spectral resolution.
Excellent — this is a very sensible question, because the gr-sdrplay3 block only works cleanly with specific, fixed sample rates supported by the hardware of SDRplay RSPduo.
If you try to use an unsupported rate, GNU Radio will either fail to start or silently coerce to the nearest valid rate — which can be confusing.
Here are the officially supported IQ sample rates for the RSPduo (as of current SDRplay API versions):
📡 Supported sample rates — Single Tuner Mode
| Sample Rate (Hz) | Notes |
|---|---|
| 2 000 000 | ✅ Very stable — recommended for spectral line work |
| 3 000 000 | Supported but less common |
| 4 000 000 | Common |
| 5 000 000 | Supported |
| 6 000 000 | Widely used |
| 7 000 000 | Supported |
| 8 000 000 | Maximum full bandwidth |
📡 Supported sample rates — Dual Tuner Coherent Mode
| Sample Rate (Hz) | Notes |
|---|---|
| 2 000 000 | ✅ Most commonly used for interferometry |
| 6 000 000 | Higher data rate, requires strong USB throughput |
💬 Only 2 MHz and 6 MHz are valid for coherent dual tuner operation on the RSPduo.
These rates ensure both tuners remain synchronized.
📡 Supported sample rates — Low IF / Narrowband Mode
| Sample Rate (Hz) | Notes |
|---|---|
| 250 000 | Good for narrowband (e.g. single H I line) |
| 500 000 | Common for low data rate |
| 1 000 000 | Mid-narrowband option |
⚠️ Low sample rates are great for reducing data volume and increasing frequency resolution in FFTs.
But make sure your IF bandwidth setting matches or is lower than the chosen sample rate.
🧠 Practical tips for 1420 MHz work
- ✅ Recommended:
2 000 000 Hzsample rate
→ gives ~1.536 MHz usable bandwidth (the rest is guard band). - For high-resolution H I studies or interferometry, 2 MHz is almost always sufficient.
- Use 6 MHz if you need more bandwidth for RFI excision or special processing.
- For very narrowband work, 250–500 kHz is fine — but make sure your decimation and FFT are configured correctly.
Summary of valid sample rates (Hz):
250000
500000
1000000
2000000
3000000
4000000
5000000
6000000
7000000
8000000
Dual coherent mode: 2000000 or 6000000 only.
If you tell me whether you’re running single or dual tuner, and what your target bandwidth is (e.g. just H I line or broader), I can suggest the optimal sample rate + FFT resolution combo for your flowgraph.
Do you want me to give you that specific combination? (e.g. “2 MHz sample rate with X FFT size gives Y Hz per channel.”)
Currently my flowchart uses dual tuner mode with 2 probes in cantenna waveguide, one at 1420 MHz and other and 1423 MHz as calibration signal.
Great setup. In RSPduo dual-tuner coherent mode, each tuner can run up to ~2 MHz bandwidth (≈2.0–2.048 MS/s) and both tuners must share the same sample rate. Center frequencies may differ. (SDRplay)
What to use
- Sample rate (both tuners): 2.048 MS/s (or 2.000 MS/s).
This is the most stable/portable choice in GNU Radio for coherent dual-tuner work.
Example for your flowgraph
- Tuner A (cantenna #1):
fc = 1420.405e6,fs = 2.048e6 - Tuner B (cantenna #2):
fc = 1423.000e6,fs = 2.048e6 - Mode: Dual-tuner coherent (shared clock), AGC off, identical manual gains.
Why this works: the RSPduo can run both tuners simultaneously with ~2 MHz per tuner, so placing A at 1420.405 MHz (H I) and B at 1423 MHz (cal) is valid even though they’re 3 MHz apart—each tuner covers its own 2 MHz window. If you tried to do both within a single tuner, the 3 MHz separation would exceed the per-tuner bandwidth. (SDRplay)
Note: 14-bit performance is maintained for sample rates ≤ 6 MS/s overall, but the simultaneous dual-tuner bandwidth limit is ~2 MHz per tuner, hence the 2.0/2.048 MS/s recommendation. (SDRplay)
If you want narrower data rates downstream, add decimation in GNU Radio after each source (e.g., decimate by 4 → ~512 kS/s) and size your FFT for the channel resolution you need. (wiki.gnuradio.org)
When you say Sample rate (both tuners): 2.048 MS/s (or 2.000 MS/s),.do.you mean sample rate of 2000 or 2000000?
Good catch — this is a very common gotcha when entering numbers into GNU Radio.
When I said “2.048 MS/s (or 2.000 MS/s)”, I meant:
👉 2.048e6 or 2000000 samples per second —
not just 2000.
📌 Correct ways to enter it in GNU Radio:
2.048e6✅2048000✅2000000✅ (if you want exactly 2 MHz)
❌ Wrong:
2000→ would only give 2 kS/s (far too low and will not work properly with RSPduo).
💡 Why 2.048 MHz specifically?
That value aligns with binary multiples (2¹¹ × 1000), making downstream decimation (e.g. by 2, 4, 8…) clean and efficient for spectral work.
✅ So for your dual tuner coherent mode flowgraph:
Sample rate: 2.048e6
Tuner A: 1420.405e6
Tuner B: 1423.000e6
…or use 2000000 if you prefer a round number.
TL;DR:
Use 2.048e6 (or 2048000) — not 2000.