Cascading LTCC Filters to Filter a PLDRO-Derived Local Oscillator
The local oscillator (LO) has been around since Edwin Howard Armstrong invented the superheterodyne receiver over a century ago, and for as long as LOs have existed, radio engineers have sought to make them more spectrally pure. The reason for this is that superheterodyne receivers mix the incoming RF with the LO and its spurious responses, harmonic emissions and noise to construct the IF output.
This article focuses on mitigating high harmonic levels and spurious emissions for both the 10 GHz fundamental frequency and the doubler output of 20 GHz by cascading tiny LTCC filter components together and combining them with MMIC amplifiers to notionally drive a level 15 mixer. Read on to find out how effective these tiny LTCC filter components can be in reducing LO harmonic content.
Higher order digital modulation schemes of today require that the LOs be exceptionally clean in every respect; spurs, harmonics and phase noise. Local oscillator phase noise is often a major driver of RF system performance. We will explore phase noise in a future article, utilizing a similar LO frequency and identical MMIC amplifiers to those found here. The future article will include the impact MMIC amplifier additive phase noise has on the phase noise of the source and the effect of integrated phase noise on system BER for various modulation orders and schemes.
Cascading LTCC Low Pass and High Pass Filters
The QPDO-E-10-101 is a Qualwave 10 GHz phase-locked dielectric resonator oscillator (PLDRO) that exhibits ultra-low phase noise at an RF output power of +13 dBm minimum with harmonic levels that are specified at -30 dBc maximum, or approximately -17 dBm in terms of absolute power level. As shown in Figure 1, the PLDRO drives a pair of tiny 0603 LTCC filters in cascade, the first of which is the LFCW-8700+ low pass which is utilized primarily to remove the 20 GHz second harmonic, a frequency at which this filter achieves 39.29 dB of rejection. The next filter in cascade is the HFCW-9500+ high pass that is applied to clean up spurious and subharmonic content in the region of the output spectrum below the carrier frequency. Both these filters come in 0603 LTCC footprints, albeit with different terminal configurations, and each has slightly greater than 2 dB of insertion loss at the 10 GHz PLDRO fundamental output frequency. The exact levels of the 10 GHz fundamental and 20 GHz second harmonic after being filtered by this low pass-high pass arrangement are shown in Figure 1.
Figure 1. Block diagram of a notional LO driver comprised of a 10 GHz PLDRO source, a frequency doubler, cascaded LTCC filters and GaAs HBT MMIC amplifiers
A Low Phase Noise MMIC as an LO Driver
Since the downstream frequency doubler requires +12 to +18 dBm of input power, the filtered 10 GHz carrier must be amplified before being applied to the doubler. The LVA-6183PN+ GaAs HBT is a 6 to 18 GHz ultra-low phase noise MMIC amplifier with a gain of over 20 dB and an output P1dB of approximately +20 dBm at 10 GHz. The LVA-6183PN+ is driven to P6dB with the +8.44 dBm RF input level which stabilizes its RF output power at approximately +22 dBm. The downside is that strong harmonic content is regenerated due to deep compression and the highly nonlinear nature of the resulting RF output waveform. The second harmonic at the LVA-6183PN+ output is estimated to be -10 dBc, or +12 dBm.
Attenuating Harmonic Content from the LO Driver
Naturally, it is undesirable to drive a frequency doubler with strong harmonic content, particularly with the second harmonic of the source, so the BFCN-1052+ band pass filter is utilized to attenuate the harmonics, with special emphasis on the second harmonic. Note that the BFCN-1052+ band pass filter is inserted between two fixed 50 GHz MMIC attenuators, the QAT-2+ and the QAT-1+. While these attenuators are included for improved return loss and to promote the stability of both the amplifier and filter frequency response, RF/microwave circuit and EM simulations are great measures to take in order to ensure the broadband stability of any LO driver amplifier.
The BFCN-1052+ band pass filter will knock the 20 GHz second harmonic down by nearly 32 dB while only affecting the 10 GHz fundamental by 1.28 dB. Including the net 3 dB loss of the two fixed attenuators, the fundamental and second harmonic levels going into the frequency doubler are shown in Figure 1. Note that the second harmonic is approximately -40 dBc, which is perfectly acceptable, and that, by virtue of the frequency response of the BFCN-1052+, the third (30 GHz) and fourth (40 GHz ) harmonics have been attenuated by 30 and 40 dB, respectively.
Doubling the Fundamental Frequency and Associated Harmonic Levels
The 10 GHz fundamental is doubled to 20 GHz using the CY2-44+ 2X MMIC multiplier which is capable of an output frequency of 12.4 to 40 GHz. Since the RF input level to the doubler is very close to +18 dBm, we used the +18 dBm test data to determine the RF output harmonic levels shown in Figure 1. With the 1f (10 GHz) and 3f (30 GHz) levels only ~-30 dBc and the 4f (40 GHz) at less than -20 dBc, clearly additional filtering is required. In this case we turn to the BFCQ-1932+ band pass filter, which affords excellent rejection for 1f, 3f, and 4f and comes in a compact 1008 (2.5 x 2.0 mm) package. The resulting harmonic levels are shown at the output of the BFCQ-1932+ in Figure 1.
The Final Push – Driving the Mixer
Not only does the BFCQ-1932+ attenuate out-of-band, but in-band, at 20 GHz, the insertion loss is slightly less than 1 dB. This enables the final ultra-low phase noise MMIC amplifier, the LVA-273PN+, with its +2.53 dBm input and 18.2 dB of gain, to drive beyond P1dB to approximately P2dB and +18.5 dBm RF output power. Just as with the first LO driver amplifier, the harmonic content (in this instance harmonics of 20 GHz) will regenerate, to the extent that the amplifier can support them. The LVA-273PN+ is specified to operate from 10 MHz to 26.5 GHz, so the 40 GHz second harmonic will inherently be suppressed. However, with significant gain across that broad a spectrum, and a bandpass filter cascaded between the CY2-44+ multiplier and the LVA-273PN+ amplifier, there is risk of instability. On the output side of the final LO driver amplifier are the same two fixed MMIC attenuators (QAT-1+ and QAT-2+) that were applied to the output of the first LO driver amplifier. Between those attenuators is another instance of the BFCQ-1932+ band pass filter that has very low insertion loss at the 20 GHz LO frequency and excellent attenuation outside the passband, all the way to 40 GHz. The final drive level of +14.5 dBm is shown in Figure 1. This is sufficient to drive a Mini-Circuits’ level 15 mixer such as the MDB-44H+.
The LO Down
In this article, we demonstrated how we could take the output of an ultra-low phase noise 10 GHz PLDRO and amplify, filter and double it using all Mini-Circuits’ parts. Once doubled, we filtered, amplified and filtered again before driving the LO. We tracked carrier and harmonic power levels along the way, eventually achieving sufficient power to drive a Mini-Circuits’ level 15 mixer.
Ultra-low phase noise MMIC amplifiers were used as LO drivers on the block diagram and the importance of generating a low phase noise carrier was briefly emphasized. A future article will center around achieving exceptionally low phase noise LOs by utilizing Mini-Circuits’ ultra-low additive phase noise MMIC amplifiers. The future article will be replete with both the phase noise calculations as well as system design considerations.
References – Links in order of appearance in block diagram:
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