Whether or not a radio is a joy to listen to, or apt for difficult DX is decided to a large part by the last IF filter stage. Most radios of the past didn’t come with many IF filters to begin with, their quality varied a lot and harsh compromises had to be made in multi-purpose radios. With the advent of digital filtering and inexpensive DSP portables, we got spoiled by a rich choice of selectivity settings that came with an idiosyncrasy I had to wrap my head around first, namely a shift in how some (digital) filters are labeled now. <\/p>\n In AM mode<\/strong> both sidebands contain the same modulation information, meaning the audio bandwidth fitting into a 9kHz (region 1 and 3) or 10kHz (region 2) medium wave channel is approx. 1\/2 of the modulation bandwidth – that’s 4.5 or 5 kHz. On shortwave, the 5kHz channel spacing is good for only 2.5kHz of audio spectrum.<\/p>\n Physical IF filters are specified by their entire (IF-) passband<\/strong>, not the audio passband. To enjoy the full audio spectrum an AM station can (usually) transmit, you need a 10kHz wide filter on the US medium wave (9kHz in the rest of the world), or a 5 kHz filter on shortwave.\u00a0 The more seasoned SWLs and hams among us are very used to that scheme, hence the prospect of a digital<\/em> filter offering a maximum of only “6kHz” bandwidth or even less appears disappointing, even though that was the standard “wide” filter on many SWL tabletops and portables of the past..<\/p>\n When I got my first typical (Silicon Labs 473x-) DSP portable I started wondering about the filter choices: The widest filter setting showed familiar “6 kHz” in the display but it sounded so much wider than that! Checking the audio spectrum, I could see that the “6kHz” filter had full 6 kHz of audio bandwidth, with the other filter settings following that scheme. In other words, the settings were named after the audio bandwidth or ‘per-sideband’ bandwidth<\/strong> and didn’t follow the customary analog filter descriptions, the “6kHz” filter setting is in fact equivalent to a 12kHz IF filter<\/strong> in the old spelling!<\/p>\n Likewise, the Belka (ultra-) ultraportables – although not built around a SiLabs DSP – use the “audio bandwidth” naming scheme too, with the widest filter being called “4kHz”. This would be the equivalent to an oldschool 8kHz IF filter and it’s actually even wider – but this naming convention occasionally leads to unnecessary concerns about “too narrow” filter choices in these radios.<\/p>\n On hardware radios the naming shift might be just a means of simplification, as far as I know restricted to SiLabs-based DSP portables and the Belka, but while some SDR softwares (SDR Sharp, SDR Uno) are using the conventional ‘IF bandwidth’ naming scheme, SDR Console is using ‘AF\/per sideband’<\/strong> again. No surprise that this is confusing – but luckily only in the double sideband modulation modes.<\/p>\n In SSB<\/strong> only one sideband is supposed to go through the filter at a time, that’s why the filter’s specified bandwidth is equal to the possible audio bandwidth anyway and the befuddling naming issue doesn’t exist.<\/p>\n While trying to make screenshots to illustrate the issue, I had the idea to generally check the filters of my radios for their characteristics. In order to show that I needed to cheat a little: Rather than trying to measure filters I can’t access individually for analysis with equipment I don’t own, I merely analyzed the audio output of the radios. While this can show an approximation to the actual filter shape, this is including every signal change happening behind the filters, for example tone controls. Lacking a signal generator, I tuned the radios to a 10kHz wide DRM signal from Kuwait on 15110 kHz to give the radios a wide, actual signal and circumvent any level-based automatic bandwidth or noise reduction shenanigans the DSP radios might employ.<\/p>\n Tecsun PL-660<\/strong><\/p>\n The PL-660 is unfortunately my only example of an (almost) oldschool analog portable, sporting 2 conventional, inexpensive ceramic filters of unknown specification and not using DSP for that. Their model numbers remind a bit of the MuRata filter codes but that doesn’t correspond with the results shown below, the “CXE455IU” could be the “wide” filter. However, at the end the difference between wide and narrow isn’t …umm… exactly dramatic:<\/p>\n It looks like the passband is the same for both settings, just the skirts seem to drop a smidge faster in the narrow setting. The practical difference between the 2 settings is nothing to phone home about but certainly more noticeable in real life than the analyzer plot suggests, and also a frequent subject of complaint about later models of the PL-660.\u00a0 But the passband looks pretty good (flat), not too much of a drop-off until 2kHz,\u00a0 3kHz is the the middle of the shoulder and the audio is ~12dB softer at this point.<\/p>\n XHDATA D-808<\/strong><\/p>\n The D-808 is a great example for the bunch of small DSP-radios that save extensive (and expensive) additional circuitry and offer a particularly rich set of (audio-) passbands for AM,\u00a0 even very unusual ones like “1 kHz”.<\/p>\n D-808 at 6, 3 and 1kHz settings (audio\/per-sideband bandwidth)<\/p><\/div>\n The passband is a little more sloped than that of the PL-660, but the tiny speaker likely keeps the impact of that in check.\u00a0 The big surprise is that the skirts are dropping so gently that even the PL-660’s ceramic filters seem to have a slightly better shape factor.\u00a0 I’m not sure if that is intentional or just a limitation of the Silicon Labs chips that these filters seem to mimic the shape of inexpensive ceramic filters, also in the next DSP radio in the group:<\/p>\n Tecsun S-8800 Given that this radio uses the same DSP chip in its 3rd IF stage, the difference in response is surprising: The slope within the passband is much more pronounced than that of the D-808 and 20 (audio-) decibels lower at the right\/upper edge. The skirt is again surprisingly\u00a0 wide for a digital filter – both is also noticeable in the actual sound of that radio, often simultaneously a bit muffled and “too wide”.<\/p>\n Tecsun S-8800 at 6 and 3 kHz filter settings<\/p><\/div>\n Belka vs. Icom IC-705<\/strong><\/p>\n Please note that the IC-705 uses the “oldschool” (IF-) bandwidth specification scheme, while the Belka is “per sideband”. Since both radios allow setting the lower\/left skirt individually, I deliberately left the lower cutoff frequency at their lowest settings on both (I like to hear the bass in both voice and music!). Unlike the DSP portables, the right\/upper shoulders are what I’d expect from an “ideal” digital filter.<\/p>\n![\"\"](\"https:\/\/swling.com\/blog\/wp-content\/uploads\/2023\/04\/Filters_Generic.png\")
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\n<\/em><\/p>\nBandwidth confusion<\/h2>\n
Demonstration and practical (kind of) measurements on some radios<\/h2>\n
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